West Hoathly Seagull
Honorary Ruffian
If we simplify it to just right and left for the purposes of an easy explanation. If everyone is slightly further right than "usual" then the swing voters haven't moved from their centrist/pro-EU position, they have swing to "slightly right" and thinking of voting Tory. The "raving right wing" Tory voters have also moved "slightly right" and are now planning to vote UKIP. It could be as simple as one-in, one-out holding the main parties support static but making it look UKIP have gained the swing voters.
Add another dimension of complexity and assume, instead of everyone shifting slightly right, they shift slight away from centre and those raging left wingers are falling off the edge of Labour into Green and its easy to see how all the "minority" parties benefit.
Before I get flamed on this I fully appreciate that political voting is simpler then just a linear scale and that a UKIP voter isn't necessarily a rightER-wing Tory and a Green isn't a leftER-wing Labourite. I was only using this to show how centrist swing voters and gains by the fringe parties aren't mutually exclusive.
I think the divide could also be between urban and suburban/rural voters and between London (and other socially similar cities) and smaller towns. The Times recently had a survey on attitudes to Europe, immigration and so on (unfortunately I didn't bookmark the link, and in any case it's behind a paywall). My constituency, Battersea, swung quite strongly Tory in 2010 (the majority is about 5,500), and most posters on UK Polling Report (www.ukpollingreport.co.uk) expect it to remain Tory next year. However, it was about the third most pro-immigration constituency in the country, and only 24% of voters were comfortable with the idea of the UK leaving the EU (different I suppose to wanting to leave it) - contrast that with Bexhill and Battle, where the figure was 67%. In addition, Sutton and Cheam, despite being a Lib Dem seat, had far lower pro-immigration and far higher anti-EU figures than Battersea (the same contrast applies with Horsham - safe Tory and Crawley - marginal Tory and Labour for all the time they were in power). My MP, Public Health Minister Jane Ellison, is also quite pro-Europe.
I am also not surprised some Labour supporters have gone for UKIP. Quite a lot of their core support outside the big cities, despite being standard left wing on the NHS and Trade Unions, etc, is rather socially conservative. If you read Tony Blair's memoirs, he describes how shocked he was at some of the attitudes he found among party members in Sedgefield. Their views on Law and Order, homosexuality and so on would not have gone down at all well in North London and other similar places.