"But people have never voted the same way at the Euros (or even the locals) as they do for Westminster, or there would be UKIP MPs.
If Westminster had a PR system, maybe?"
Exactly, it is only because of our voting system which is unjust and unrepresentative. Natalie Bennett of the Greens said our system is "broken". It does not represent the true wishes of the people. It will have to change sometime, it is just a question of when.
"So in the absence of any game changer, the most this will do is split the Conservatives - and the more high profile Tories defect to UKIP, the less attractive they will be to disaffected Labour supporters."
I think you are underestimating the disaffection out there. The Tories will leak more votes because real Tories don't believe Cameron is a Tory ans as UKIP's polling rises, Labour voters will switch to UKIP because they don't like the metroplotan elite PPEs who run Labour. At the moment the news is telling us that Miliband is being attacked by some of his own party. But I think the real panic will start to set in in teh Tory Party, because the tories will not be able to win their disaffected voters back whereas Labour might.
Then the Tories will have to do a deal, and Farage has alredy told the world that he is ready to deal with the devil, so a deal will be done and if it is a deal that takes the best UKIP policies, then Labour will use, because they can't out UKIP UKIP.
Will the Etonians swalllow their pride? They will have to, because they will be told to by their financial backers when they start reading the polls.