DISCLAIMER: I'm not British, I don't vote, and I don't plan on living here forever, so my observations are those of a dispassionate outsider.
I've met a fair few Lib Dem supporters and they fall into 2 groups. There's the vast majority of "opportunistic" Lib Dem voters, who voted for them in the last couple of elections because they were hugely disappointed with Labour's pro-big business, authoritarian policies. Those guys probably won't vote for them again any time soon.
Then tere's "hardcore" supporters, of whom I've met fewer, and who seemed to me to be a weirder bunch: they tend to be well-educated young people from well-off middle-class backgrounds, whose friends are all Conservatives. Why they themselves vote Lib Dem varies, but I've yet to meet one of those who genuinely seems to wholeheartedly espouse left-wing politics. It just seems like some of them would be happier voting Conservative, but won't either becuase of inverse snobbery or middle class guilt, so they think they're sticking up for the common man by voting lib dem. I have no doubt that those many of those people are ideologically closer to Conservatives than to Labour (some of them hate Labour supporters with the fire of a thousand suns!), but going by the poll numbers they seem to constitute a minority of hte people who voted lib dem in the last election.
Sad fact is that the lib dems are just another party, led by cynical politicians just like the other two, and if you guys want to change the politics of your country your only choice is to join a party and try to change it from the inside.
If you really don't see yourself doing that, then instead of not voting at all I would suggest that you go out and cast your vote for one of the true alternative parties, someone like the Greens or something. Yeah it's a wasted vote in the short run, but in the long term if these small parties start gathering 8-9% of the vote the injustice of them not being able to get a single MP elected due to this tyranny of the minority system you've got going will become more apparent, and you might get another chance to change it in a few years' time.