GTR does seem to try hard to keep units on specific routes, which does at least mean some get hit harder and others stay relatively clean. The trains to Sevenoaks get wrecked almost every day. It's not just graffiti; the glass for the egress handles get smashed, the first class antimacassars get taken off, there's graffiti (sometimes in lipstick) and etching, stuff poured on the floor, drink, food and even clothes strewn all over the place - and that can be on the morning services.
What's maybe a little more worrying is that some seats get carved up and parts of the seat back or base is cut out.. maybe for some TikTok trend or something. Thing is; that would suggest users are carrying knives with them.
They end up like a train that's been in service all day and ran the last train out of a city, full of drunks, yet can get like that after just one run in service!
Looking at the toilets, they're usually smeary and show that some attempt has been made to remove the paint - but whatever solution is being used can't clean it off sufficiently. You end up with a pink/purple smeary mess, and soon after more people add to it anyway. I have no idea how they can solve this problem as clearly not all routes are bad. The 717s into Moorgate have some tagging, but nothing like the same (obviously no toilets to mess up - and the egress panels have a solid cover).