If they don't vote as their constuency as a whole wishes, will they be elected the next time round? They can also bear in mind all the conflicting desires of individual constituents, even if they didn't vote them into the seat, but their vote is supposed to be representative of the voters they represent.
A party is supposed to a group of MPs, all voted in at the wish of their constituents, who have a common aim, directed by the wishes of those who sent them to Parliament to represent them. That is what democracy is all about.
The operative word is "represent" I wouldn't ask someone to represent me at as much as a vicarage teaparty if I thought that they would speak and act in a way that didn't correspond with my wishes.