Hold a by-election whenever a sitting MP changes political party. — 7 signatures. Status: rejected.
People vote for an MP because they believe in a particular political party or manifesto, if that MP then changes their party they should stand again in a by-election. This would ensure that the voters are represented by the party they want rather than by the whim of a particular MP.
An MP voted in on a particular manifesto/political party who then goes on to change their allegiance to another party is no longer representative of the voters in their constituency.
View this petition on the official UK Parliament website