What explains the huge difference between UKIP's high-watermark in EU vs UK elections?

Question

On one hand, UKIP came 1st among British parties in the 2014 MEP election with 24 seats, representing 32.9% of British seats. They even came first in the popular vote in that contest with 26.6% of votes (slightly ahead of Labour, 24.4% and 23.1% Conservative).

On the other hand:

Ukip has only ever had two MPs, and currently has none - its only remaining MP, Douglas Carswell, left the party in March 2017, and told Sky News’ Adam Boulton that there was “no point” to the party anymore.

I couldn't immediately find out their high-watermark of popular vote in a national election; maybe that can be part of the answer. In the 2015 election they apparently won 13% of the popular vote; that might have been their high-watermark in a national election, but I'm not entirely sure.

My question is: what explains this large difference between UKIP peak results in EU vs. national elections? (And I mean at least in terms of seats, but also popular vote if I got the high-watermark numbers for the latter right as well.)

Answer 1

There are a few real or perceived effects going on here:

1. Elections to the European parliament have previously been described as second order elections, rather like local elections or the US midterms, with lower turnout, and more voting to "send a message" rather than to choose leaders. In particular, they haven't been very favourable to parties in government (note the collapse of the Liberal democrat vote, the junior member of the coalition).

2. This effect was likely exacerbated by UKIP's near single issue mentality of withdrawal from the EU, and voters effectively treating the election as a pseudo referendum on EU participation.

3. Different voting systems are used in the two elections, with general elections using single winner districts chosen by first past the post (plurality) voting, while the European parliamentary elections used multimember, closed list proportional system in England, Scotland and Wales (the D'Hondt method) and STV in Northern Ireland. This meant that a relatively small difference in vote share lead to a very significant difference in seats won.

Answer 2

The simplest answer is that UKIP (United Kingdom Independence Party) voters is that European Union elections have different policy impact than UK elections. In the UK elections, these UKIP voters may prefer the policy preferences of one of the other parties for local, domestic matters. Further, in local, domestic elections both the Conservative and Labour parties are now officially anti-EU. In previous elections, the local candidates might have been officially anti-EU, even though the party itself may have been neutral or pro-EU in aggregate. So in those elections UKIP voters get the UKIP policy without having to vote for UKIP.

In EU elections, I suspect that only pro-EU candidates run to be EU Members of the European Parliament. Or even if they are somewhat anti-EU, they end up taking some effectively pro-EU positions because they match domestic principles. UKIP would oppose legislation at the EU level, even if they agreed with the point of the legislation. That kind of position meant much less domestically.

As noted in a comment, part of this is also the first-past-the-post system. In the UK, voters have to choose between voting their first preference and voting for the person most likely to make it. In the EU, the proportional system means that voters can get their first preferences. This means that domestically, voters who preferred UKIP may have compromised on other parties. In the EU, they don't have to do that. They can vote their first choice and rely on the voting system to make that work.

Both reasons (and probably others) can be true at once. UKIP's single policy can have more impact in EU policy than UK policy, and the voting systems may allow the EU representation to more directly reflect voters' intents.