Obviously when a party goes from 0% to 28% - some of that support will come from each of the older parties - but again - if you are the Labour Party you have to drool at the thought of all those Lab/Tory marginals where if UKIP goes from 0% to - say - 15% - 3 or 4 points will come from Labour while 10-11 will come from the Tories - all in all the rise of Ukip is good news for Labour in that for the most part it splits the rightwing vote....and in the longterm if the rightwing vote in the UK gets split between the established right (ie: Tories) and the populist, eurosceptic right (ie: UKIP) - its a boon to the Labour party.
Mitterrand tried that thinking here back in the 1980s. Now Hollande is not amused...