How long has he been in England? He does seem to have moved around quite a bit in his short life, and from being a feral to an indoor pet thats alot in 2 short years.
As has already been suggested, keeping bedroom doors shut so he doesn't have access may help break the habit. Urine Off
R & L Pet Products - cats, breeders, shows, litter, food, and accessories. is another good product you could try to remove any smell on your bedding and the carpet.
The spot on the carpet he frequents you could try putting his litter tray over it. The same place that sells the urine off also sells Cat Attract a cat litter with "additives" that is supposed to encourage a cat to use it
R & L Pet Products - cats, breeders, shows, litter, food, and accessories.
If you are seeing your vet anyway, I would ask him about possible low grade urinary infection. He may have a chronic condition that can be caused by stress. Some times inappropriate weeing as opposed to scent marking are an indication of this even if there are no other discernable symptoms. The cat feels uncomfortable weeing which they end up associating with the tray, so find somewhere softer to try. If it is something like that then treatment and maybe changing the tray and litter so there are no old reminders could help sort the problem.