What your describing is the party whip system. High time MP's were allowed "free" votes on all issues, instead of being told how to vote by their inglorious leader.
So why don't MPs vote the way their constituents want? I know an MP can lose the 'whip' but what does that mean to the individual MP?
I don't fully understand the whip system either, but I presume that if they don't follow the party line they may not receive any support from their party at the next election. Perhaps there are other consequences too.