First past the post means a party with diffuse but high support gets very little in the way of seats.
Most European countries use PR because its more democratic and ensures every voter’s vote is not wasted.
Yeah, PR worked very well for small-d democrats in the Weimar Republic. PR leads to political polarization and extremism.
It's also worth noting that UKIP's better performance in the EP is also a result of better election performance in EP elections than UK parliamentary elections. UKIP received more votes in EP elections than any other party -- more than the Tories or Labor. A fair conclusion is that UK voters want to express more Euroskepticism in the European Parliament than the House of Commons. Perhaps domestic issues outweigh Euroskepticism in House of Commons elections.