I reckon UKIP will simply become a slightly more to the right Conservative Party who when confronted by the fact that being too right wing is off putting to the electorate, will mellow out to the position I mentioned above.
We had a party in Canada similar to UKIP, it was called The Reform Party but even though it had a solid following in the western provinces, they could never convince the eastern more populace provinces to back them. their future therefore was that they would have spent eternity sitting as the opposition party to the Liberals.
The conservative party by this time had been decimated to the point where they did not have sufficient members to be recognised as an official party, thanks to the total f*ck up made by their leader, a certain Brian Mulroney. End result, The Reform Party toned down some of it's rhetoric and wooed the Conservative Party into joining them then took on the name of the Conservative Party and after a couple of minority governments, they finally got themselves a majority the last time around.
They are still a bunch of Reform wankers in my book and may or may not be successful in the next election but hey, if it's the will of the people then that's what you get.
I believe UKIP will do something similar should they manage to grow but not to the point where they have a chance in a General Election, I think you'll find there will be some compromises made in return for their support of the Conservatives.