Plenty of ex-Labour voters went to UKIP as people felt the party had abandoned its base in favour of metropolitan liberals.
Not completely fair imho, but it's also got more than a grain of truth.
Most of those people, it seems to me, will have swung to the Conservatives but it won't be them that swings the election - UKIP support was always substantial as a sheer number but was thinly spread across the country, which is a death sentence in the UK's dumb voting system.
Historically speaking, the two most powerful influences on UK elections are the Murdoch-owned press which are rabidly pro-Tory and the grey vote which will probably stick with the Tories albeit with less enthusiasm than before. If Labour does manage to really significantly increase turnout, it would mean a fundamental change in the voting pattern which has dominated my lifetime. Not impossible by any means, but unlikely.