The MPs say the current first-past-the-post system for general elections is "broken".

As far as I can tell, this is referring to this proposed amendment of the Representation of the People Bill:

https://bills.parliament.uk/bills/4080/stages/20676/amendments/10035247

So, not quite a petition for getting rid of FPTP per-se, but proposing an independent body is set up to determine the fitness for purpose of the current system, and suggest ways to make it fairer.

There’s a little reading-between-the-lines needed.