Most the voters that gave the Lib Dems 63 and 57 seats respectively in 2005 and 2010 were Labour supporters protesting Blair and the Iraq War and fashioned the Libs as a viable center-left alternative. They were certainly not happy to have in effect voted to put up a Conservative government.
If that was true the Labour popular vote would be something like 3 million up on 2010 and they would have won this election in a landslide.
Most lost Lib Dem voters seem instead to have gone to UKIP which would indicate they are general protest voters and not Labour voters.
I don't quite know why you think people voting for the Liberals who are the most euro-friendly of all of the major parties would suddenly switch to UKIP. Sure, the Liberals since the 1970s have taken their fuel from protest votes, but those voters in the last two elections were protest votes from the left who did not want to have the government that they got over the last five years.
All of those people were certainly not true liberals in their hearts and can't be descried as part of the center ground that Clegg fashioned himself the leader of.
