I agree voting doesn't really change things, not that I can't imagine not voting does any particular help either.
That said... I mean, I suppose voting probably isn't worth the effort, so... eh, maybe you shouldn't bother.
I'm speaking from the perspective of US politics of course, not British politics.
I suppose if you do at least vote, it'll at least tell them what positions the should probably support and oppose to stay popular. Even if real, important change doesn't come from it, it at least means, for instance, politicians with radical homophobic positions not getting votes means those who do get elected will know to oppose those positions, at least enough to keep people complacent.
At the end of the day, though, it's all a nice big stage production, and we're just helping to chose the actors.