Members of Parliament can vote, or not vote, however they like. If they don't toe the party line, then under the whip system, they might face the sanction of being suspended or expelled from their party. However, that doesn't make an abstention unlawful, and it doesn't prevent the MP from continuing to sit in Parliament as an independent.
There is no option in the UK to vote "present". You either vote "Aye", "No", neither, or both (the latter two options both being equivalent to an abstention). There's a related concept called "pairing" where an aye and a no voter both agree to abstain so that they cancel each other out. This is normally used when one can't attend parliament (e.g. because of illness).
