The only thing that bothers me about a proportional system being introduced in Britain is that the share of seats will be split among three parties and not two. It’s perfectly possible that the two centre-left parties could form a permanent (but unofficial) coalition that sees them both staying in power as long as they can agree on most policy. Which is more likely than either Labour or the Lib Dems agreeing to form up with the Conservatives (the only reason it’s looking like it may happen this time is because it’s the option that poses the least risk for Clegg).
And as long as both Labour and the Lib Dems can manage to maintain their core vote (one illegal war, a damaged economy and spiralling debt on their watch later, and Labour still gets 8.5 million votes…) then there would be no way to get a change and we’d effectively be living in a one-party state. That kind of situation is what leads to corruption. And it’s hardly democratic.
