Given the three major parties, and the fact there isn't a party with seats to the right of Cameron, I'm not sure what difference he could have made. Sure, he's not very conservative, but NO ONE voted for Labour or the LibDems because Cameron wasn't conservative enough.
The fact of the matter is the clear majority of the voters in the UK actually want as much socialism as they can get. This is compounded with the problems that come with multiculturalism. Cameron is about as far to the left as he can be and without actually being in Labour. He lost no votes on his right because of it. Seems to me, he - and the Tories - are in a lose or lose really big scenario. He managed the former, not the later, which I guess is a bit of a victory.
Labour is your typical big government leftist party similar to the democrats in this country. Libdems are actually a combination of two parties the Liberals and Social Democratic. The liberals were roughly equivalent to the Libertarian party in the US and the Social Democrats were basically socialists. A strange combination! The closest thing I could think of in the US would be if the Libertarian party and Ralph Naders group joined forces. They are general considered a centrist party. Although they are farther to the left than Labour on some issues and farther right than the conservatives on others.
It's not quite true that Cameron had no competition to his right. Perhaps they're akin to the Birchers Bill Buckley wisely purged from our right, but there is a party, which claims to be to Cameron's right and may have cost him seats, the UK Independence Party Although it didn't win any any in the House of Commons this time, the UKIP does have two seats in the House of Lords and, if Wiki is correct, some in the European Parliament. I've seen claims that had the 3.1% of the vote the UKIP drew this time gone to Cameron's party the Tories would have won the majority. With the proportional representation the UKIP want they'd have won 20 seats on their own. Of course with that the Tory's tally would have been 70 less seats, so you have to be careful for what you ask! I'm no expert on UKIP, other than knowing they've been accused of lots of things by the liberal press, but I'm not aware of them being accused of being leftists. So there is at least alleged rightist noise, if not seats, on Cameron's right.