UK Parliament question

I was reading this story and I got to wondering. How does the party leader enforce an ordered vote? What happens if a lawmaker votes in defiance of party orders?

The party line is enforced through the whip system - whips are elected MPs who are selected by their party leadership to encourage MPs to vote a particular way.

Lots of information here:

The duties of Whips include:
[ul][li]keeping MPs and peers informed of forthcoming parliamentary business [/li][li]maintaining the party’s voting strength by ensuring members attend important debates and support their party in parliamentary divisions [/li][li]passing on to the party leadership the opinions of backbench members.[/ul]The term ‘whip’ also applies to the weekly circular sent out by each Chief Whip to all their MPs or peers notifying them of parliamentary business. The degree of importance is indicated by the number of times that the debate or division is underlined:[/li][ul][li]items underlined once are considered routine and attendance is optional [/li][li]those underlined twice are more important and attendance is required unless - in the Commons - a ‘pair’ (a member of the Opposition who also intends to be absent from the division) has been arranged [/li][li]items underlined three times are highly important and pairing is not normally allowed[/ul]‘Three-line whips’ are imposed on important occasions, such as second readings of significant Bills and motions of no confidence. Failure by MPs to attend a vote with a three-line whip is usually seen as a rebellion against the party and may eventually result in disciplinary action, such as suspension from the parliamentary party.[/li][/quote]
Suspension from the parliamentary party is known as “withdrawal of the whip” or “having the whip withdrawn”. Enforcement is via threats of obscurity, promises of promotion, murmurs of being in the leader’s favour and even blackmail - reputedly.

(note: whips are only elected MPs in the House of Commons; sorry)

The main sanction the party leader has against those who don’t follow his/her line is that he/she can refuse to promote them. This isn’t so effective if the party is in opposition as the party leader has (as yet) only limited patronage, but his/her patronage will be considerable if he/she becomes Prime Minister. Unsurprisingly, Prime Ministers tend to favour those with a record of loyalty.

Each party has some of its MPs designated as whips. They are responsible for party discipline, especially ensuring that MPs vote the way the leadership wants them to. They cannot force people to vote the way they want ( MPs are officially elected as individuals, not as party representatives, and can vote however they want. Their main weapons are threats: people can be advised that voting against the party line will seriously reduce their chances of getting a government post in the future. The most extreme sanction would be withdrawal of the party whip. This would make it impossible for the person to stand as a representative of that party at the next election, making it virtually certain that they would lose the seat