Yes,they do.However I have seen one blessed suggested that UKIP will win any more than a few seats in a General Election.OTOH,they could easily take enough votes from Conservatives in a marginal constituency to give Labour the seat with 35% or 40% of the vote.
Remember....BillyBobBlythe 43%...Ross Perot 20%.
Your comment indicates you don't understand parliamentary forms of government in general and Britain's system of gov't in particular. Britain doesn't have an electoral college which results in the presidential candiate who gets an absolute majority of electoral votes becoming President. If we had had a parliamentary type of gov't then Perot would have been kingmaker and would have cut a deal with either Bush or Clinton to make one of them President, depending of course on who offered him the sweetest terms.
In the case of your scenario, where the Tories and UKIP got a majority of parliamentary seats, then they would almost certainly form a coalition. That's a much more plausible scenario than Labor Party cutting a deal with UKIP to put Labor in charge.
Good grief. Only idiots will trot that out to try to argue prima facie against independent/3rd party runs.