Three line whips exist as a mechanism to maintain discipline and to ensure vital government legislation gets through. I can understand your way of thinking, but if candidates stand for election to Parliament on their party's manifesto, it's not unreasonable once elected to be compelled to vote for those things the electorate put you there for.
If, on soul and conscience, you choose to vote against your own government they'll simply give you a verbal toasting or withdraw the whip and end
your party membership. You'll remain an MP, obviously, but will be without a party framework with which to stand for re-election.
I can think of more important issues for the United Kingdom Parliament than the whip system.