I've always found that interesting. In the US, we definitely have party loyalty, but the UK government seems to be build AROUND the parties. If I'm correct, doesn't voting against your party (as an MP) in the UK government basically equal expulsion?
yes it is, very much so in fact. The main two are Labour (centre/centre-left) and the Conservatives (centre-right/right), with the third party of the Liberal Democrats (flip/flop) who currently share the majority with the Tories.
There are many other parties, some 4,000 odd the last time I looked, but they really matter not. I suppose the far right wing parties seem to have had a little support over recent years, but their xenophobia doesn't sit well for most of the country, so they will only ever be a marginal concern.
As for the votes, it wouldn't necessarily lead to an expulsion, and I believe it depends on the nature of the vote at hand. If it was very important, such as the Iraq war, then yes, them MPs would have been threatened with a 3 whip attendance demand. Normally, rather than voting in opposition, MPs would often simply not attend the vote. However, in the case of a 2 or 3 line whip, non-attendance at the vote could lead to expulsion from the party.
The whip, in a sense, is the MPs connection to the party. Withdrawing the whip is the biggest punishment one could receive as it basically leaves you, as an MP, without any say in Parliamentary matters and your party not giving a damn about your opinion.
The system is archaic and needs to be removed. It basically stops any MP from being rebellious - much like most political systems: they're simply their to make sure the politicians serve the system, rather than the electorate.