MPs should not report criminals

Global Justice has made a petition, asking MPs to pledge to not report illegal immigrants, publicly stating who has and advising people not to seek advice from those who haven't.

It is claimed that "MPs have a responsibility to advocate for all their constituents, regardless of immigration status", which I'd agree with (as a big picture) but would dispute what qualifies someone as a constituent. You can't just turn up somewhere and demand your MP not report your crime but also to view you the same as a long term, law abiding member of the community.

The arguments used are intended to apply to illegal immigrants, but actually portray the MP as some sort of priest or doctor. Your MP is supposed to represent the constituency as a whole, which may have several conflicting priorities. Unless your life is at risk from being deported (and the Home Office will look into that), your MP is going to be weighing that up against the good of the constituency with potentially a ton of illegal, below minimum wage/unemployed/criminals wandering around invisible to the system. This isn't to say that all MPs must report (though their real constituents will make a decision on that), but to call on party whips to stop them is too far.

What do you think? Should you be able to confess to your MP without being reported? Or should they be able to use their judgement of what is best for the constituency to report or not?