Yes, it can do. Without the real threat of UKIP taking votes from the Conservative Party in the UK, David Cameron would not have promised the in/out referendum on EU membership.

This is a fair counter example, but one also has to consider that (i) much of the referendum demand came from within his own party for ideological rather than practical reasons and (ii) he was able to implement it because the UK's electoral system and multiparty politics gave his party a clear majority with a minority of the vote .

Or put more simply: his personal standing within the party was threatened more than his party's electoral chances (UKIP votes were irrelevant in most Tory seats and took votes off Labour too in marginals), and he wouldn't have been in power anyway under a proportional system.

Ironically, EU Parliament votes being proportional probably played a major role in Farage deciding back in the nineties that he could try to win the Brexit debate from outside the Tory party too...

The votes were not irrelevant. Most people think that offering the referendum to UKIP voters is the only reason the Conservatives weren't forced into another coalition government.

It was more to do with appeasing the backbenchers in the 1822 Committee than voters.

UKIP went from having a 3% share of the vote in 2010 to 12-13% in 2015. People constantly downplay this to make it out as if it was only an internal Conservative party dispute. However there was growing pressure to have the referendum.