In mid term elections there is usually a protest vote against the sitting government.
This time of course it's unusual because two parties are in government.
The interesting thing is that in the past when the Lib Dems were a party of protest rather than a party of power people voted for them with no real idea what they stood for. Slowely they changed a lot of that protest vote into permanent votes that allowed them to win a lot of councils, councils where in theory they had no right to be in power (Sheffield Liverpool and yes even Chesterfield spring to mind)
The difference now is that UKIP are the vehicle for peoples protest votes but it will take far less time for them to get their message over of what they stand for, in fact I think most people who voted for them already know.
The big worry in the past has been that Independance for Scotland (and indeed Wales) would make a Labour government unelectable in England. Also a worry has been that boundary changes in England would achieve the same thing. If UKIP does establish itself as the 4th or maybe even 3rd party if English politics then the Tories will have things far less their own way.
I can see a situation where UKIP and some disaffected Tories would occupy the right, the rest of the Tories the Centre Right and Labour the centre Left with who knows maybe the majority of the Lib Dems returning to their SDP roots and returning to a centre left Labour party. Either way that still leaves the traditional working class with nowhere to go.