When an MP resigns or leaves the political party that they were elected as a member of by their Constituency Voters, a by election should be required in that Constituency, as soon as possible, to give the Constituency Voters a choice of who they want to represent them.
You may be interested in these active petitions
Currently, the situation is that; the Voters elect a Candidate (individual) MP to represent their Constituency in Parliament. However, generally people think that they are voting for a certain Party Representative, not an individual. If that Representative then decides to 'jump ship', or leave that Party for whatever reason, they can join any other Party to represent their Constituency, until the end of the current Parliamentary Term. This is wrong and the law needs to change.