venta

My guess is that a lot of people see euro and local elections as basically irrelevant (look at the way turnout is always much lower than for general elections). Therefore they feel free to use them to send messages to Westminster (which they do care about). "The government is a load of rubbish" and "we don't like Europe" are both popular messages; I'd guess that a lot of BNP votes are trying to say "we want less immigration". I'm sure the fact that UKIP regularly does well in Euro elections has a measurable effect on the policies of the major parties on Europe; in that sense voting UKIP is a rational move.

(Also FPTP means voting UKIP in a general election would be pretty pointless and quite possibly actively harmful if the lot you didn't want got in due to split vote.)