A petition to recall an MP is opened in three situations and a by-election forced so long as 10% of the electorate sign such a petition. However, if an MP changes party during a term of office there is nothing the constituents can do. We think there should be an automatic by election.
You may be interested in these active petitions
The reason is that since the electorate voted that MP in as a representative of that party, if they stood for one.