One reason why the UK has more parties than the US is simply that it is cheaper to compete in the political game.

General election campaigning is effectively limited to four weeks before the election date in the UK, not two (or more) years. The maximum expenditure permitted by an individual candidate is either £10,000 or £16,000 depending on the number of voters in the constituency and whether it is rural or urban (obviously campaigning in a rural constituency will incur more miles of travel).

The maximum that a party can spend nationally on campaigning depends on the number of seats it contests, but if it contested all 650 parliamentary seats the limit is just under £20m.

The total campaigning expenditure in the 2017 UK general election by all parties and candidates, as monitored by the Electoral Commission, was £39.1m.

The total campaigning costs for European parliamentary elections in the UK are lower than for general elections - typically about half as much.

Note, these are the cost of campaigning only - the administrative costs, funded by general taxation, are higher than the campaigning costs, and estimated at £140m for the 2017 general election.

If US readers find those numbers incomprehensibly small compared with the total cost of a USA national election measured in $bn not $m, they are not typos!

TL;DR: Its complicated

• Most of the time England has Labour and Conservative as the two main parties, with the Liberal Democrats getting a few seats and hoping that they will hold the balance of power.




• Wales and Scotland have these three parties plus their own separatist parties (Plaid Cymru and the Scottish National Party) which are broadly left and tend to vote along with Labour.




• Northern Ireland has its own set of political parties based around its ethnic divisions. In practice the Unionist (i.e. protestant) parties vote alongside the Conservatives on most things (and right now the Democratic Unionist Party are the coalition partners propping up the current government). The Republican (i.e. catholic) Sinn Fein party refuses to take its seats in Westminster on the grounds that the British occupation of NI is illegitimate.

(Aside, in response to comments. The last time the Lib-Dems held the balance of power it didn't work out well for them. The Lib-Dems were closer to the Labour party ideologically, but the Conservatives had substantially beaten Labour in seat numbers and the popular vote. Hence the Lib-Dem leadership felt they had to give the Conservatives first refusal on a coalition. They got their longed-for referendum on replacing the First Past the Post electoral system (which subsequently failed), but in return they had to abandon a major campaign pledge about university tuition fees. Voters have punished them for this ever since, and they are hoping that the Brexit crisis will be their rehabilitation as a serious party).

And now for Brexit

However right now the UK is in a major political crisis. Neither of the two main parties has shown itself able to handle Brexit. The Conservatives have made a total hash of the last three years. Labour has avoided making a total hash merely by being out of power; they are just as divided over Brexit as the Conservatives and there is no reason to think that they would have done any better.

This is because the Brexit issue is orthogonal to the traditional left-right issues that normally divide the two main parties; the position of a person on the left-right axis says almost nothing about their position on Brexit. Hence new parties have sprung up with their primary positions being on the Brexit axis and saying as little as possible about their positions on the left-right axis (for instance the Brexit Party refused to issue a manifesto).

The Liberal Democrats have taken their traditional pro-EU center-left position, but now their campaigning is emphasising their EU policy rather than their center-left.

The Green party used to be far left and anti-EU. They have now become a green-tinted version of the Lib Dems.

It is starting to look like Brexit will be the defining issue of the next General Election. At the same time there is widespread distrust of both Labour and Conservatives. Hence it is possible that we will see a complete realignment of British politics in the next parliament, with no party holding anything close to a majority and lots of tricky negotiations to put together a coalition between parties with very different priorities.

Parliament so far has been unable to get a majority vote for any Brexit option. In this it seems to reflect the population as a whole; none of the three main options of "Leave (no deal)", "Leave (deal)" and "Remain" has a majority in opinion polls or the EU elections.

So it looks like being a fascinating show to watch. I just wish I could be watching it from a different country.

This probably deserves a more complete answer, but what you have quoted is about the European Union parliamentary elections. Those use a different electoral system in the UK (more proportional) compared to their national elections (which use first past the post - FPTP). So smaller parties (like the Brexit party and formerly Ukip) can make inroads in the EU elections.

Outside of that EU elections context, there are more reasons why the UK has more parties, including devolution of power, with regional parties being more successful in Northern Ireland or Scotland. But you haven't mentioned those parties...

As for coalitions, they do get formed. The Conservatives presently run a coalition with a Northern Ireland party. In Cameron's time, they ran a coalition with the more centrist Lib Dems. Actually a really good question is how the libdems have survived in a FPTP system. There is actually a somewhat influential theory that the proportional representation came about in most of (continental) Europe because the "old left" liberal parties saw the writing on the wall with appearance of socialists, in the decades around the end of the XIX century.

The UK has three major parties that have governed: The Conservatives, Labour and Liberals. Originally the UK had two major parties the Conservatives and the Liberal party. However during the 20th century, the Labour party eclipsed the Liberal party in popularity and relegated it to 3rd place.

The Liberal party subsequently merged with the Social Democratic party to become the Liberal Democrats.

So the UK has more political parties because the Labour party replaced the Liberal party as the opposite party to the Conservatives, but the Liberal party never truly died, it simply became less popular.

The other aspect is that the UK is a union of four countries. Both Scotland and Wales have nationalist parties (the SNP and Plaid Cymru respectively) and Northern Ireland has a completely parallel set of parties to the rest of the UK.

This just leaves the Greens, UKIP/Brexit parties, but none of these parties do particularly well in general elections, the Greens have 1 seat and UKIP have held at most 2. This is due to the lack of proportional representation in general elections.

As others have noted, the difference between UK/USA and elsewhere is the First-Past-The-Post electoral system instead of PR. Which accounts for the dominance of two parties in the USA. So why is the UK different?

One difference may be that UK constituencies are smaller (around 70,000 people). This makes it possible for local factors to affect the result of an election. Occasionally, even independent candidates get elected as MPs, after campaigning on a local issue which both or all major parties are ignoring or taking the same opposite view. It also allows smaller parties to have a regional power base, most notably the Scottish and Welsh Nationalists, and the Northern Irish parties.

The other is that the UK is in the middle of a political earthquake, and quite possibly headed for a complete constitutional crisis. This has two causes. One is Brexit, an issue which cuts across both major political parties roughly 50/50, and which must be resolved in some way on a short timescale. The other is a general perception that both major political parties have come to see themselves as a ruling class, that merely pays lip-service to democracy, accountability, and their constituents' views.

The last time anything like this happened saw the decline of the Liberal party and the rise of the Labour party in the early 1900s. The Liberal party never recovered and finally merged into the Liberal Democrats of today (who are gaining much support as a party which oughtright opposes Brexit, rather than being split). We also have the newly formed Brexit party, which unsurprisingly takes the opposite view on this issue.

I'm starting to appreciate the old not-Chinese so-polite curse "may you live in interesting times".

One thing I want to add as an answer that many people seem to miss when this kind of question comes up is that the other parties in the U.S. besides the Democrats and Republicans often don't show up until the Presidential Election every four years. In the U.S. Congressional District I live in during the last mid-term election last year there was a "third-party" candidate who based all of his campaigning on a few sign posted, and a Twitter feed that was almost completely about cartoons complaigning about the other parties, rather than explaining his positions, or how he would vote or why he would be a better choice. This was one of the "national" third-parties, but they spent no money on his campaign and provided little effort to engage the voters that wouldn't already be voting for him.

If you have a party that only shows up for high-profile elections, doesn't put in any effort between said elections, and doesn't bother trying to build their party up from the local level, why would any kind of majority support it?

I see a lot of answers here, focusing on small details that are interesting, yet not very relevant in my opinion.

The question you asked is a bit too restrictive, highlighting two systems only. The fact is, the UK is not the system with the most parties. Belgium is an example of a country that has numerous parties; so are Italy or Germany.

The explanation of these differences between them I could give is the weight of proportionality in the electoral systems.

Examples :

• In Belgium, the elections are fully proportional, and coalitions between the numerous parties govern.


• In Israel, the elections are proportional, but the winner of the election must be part of the coalition that will govern, which introduces some anti-proportionality in the system.


• In France the elections are organized in two turns, without proportionality. The country is divided in zones, and each zone elects one deputy. It allows sometimes some new challengers emergence, but it is quiet rare if you look up for the past 60 years.


• The UK electoral system is quiet bizarre, with parliaments for all nations except England (Wales, Scotland and Northern Ireland) and a UK parliament. Nonetheless, the electoral system is comparable to the french system with one turn only.


• Independently, the European Union parliament has its own system of vote, which is kind of proportional, but with a high threshold to get one deputy which condemns the very small parties. Coalitions exist at European scale, but only serve as information hub for deputies, which allow them not to be required to know everything on any subjects. This is the system that allows so much parties, not the UK one.
  


• In the USA, it is a non-proportional and one turn system that is implemented. It lets zero chance for challengers to pass through the electoral system.

I hope it helped you to understand the different shades of proportionality in electoral systems.