There have been other female, working class, northern ministers (some Tory) - Margaret Beckett, Sayeeda Warsi, Justine Greening - Rayner is not the first or some special unicorn who must be put on a pedestal. All politicians in high office including the male, posh and southern will have their dealings interrogated to look for dodginess as that is a weapon for the opposition to use against them and makes the public lose trust in them. Also, as leaders who make decisions that affect everyone else, they need to behave transparently and impeccably, hence the ministerial code.
Of course there’s corruption and some get away with more than others but being seen to be on the take/avoiding tax/breaking rules is one of the things that makes politicians unpopular and tends to bring them down, as has happened to loads of ministers of all parties, classes, both sexes etc.
Also are you saying someone must be above reproach or investigation just because they’re female, WC, northern etc? That’s ridiculous and a recipe for corruption. Just because I’m a woman and a feminist doesn’t mean I support all the behaviours of all females, and if you think about it you probably don’t either.