@jamesh I don't disagree that multi-member seats are better (which we have in the Scottish Parliament, though with some odd quirks).
But, even with single-member seats preferential voting will reduce the need for and power of parties to apply so much discipline and also reduce the penalty for parties splitting.
The Tories got such a large majority of seats in the last UK general election because they got more votes than each of Labour, Lib Dems, Greens, etc, individually in quite a few constituencies but nothing like an overall majority of votes. It seems likely more Lib Dems would put Labour as their second preference and vice-versa so in a preferential voting system maybe a Labour or Lib Dem candidate would have won in quite a few of those seats.
The Lib Dems are, at least in part, a split from Labour via the short-lived SDP.
But, yeah, a simple change of voting system keeping single-member constituencies wouldn't immediately bring a change of culture but it would have opened the way to further voting reform and longer-term culture change.
@cstross