Recently (from 1st October 2022), the old 20 and 50 GBP bank notes are not valid anymore.
From what I understood, it is possible to exchange them (among others) at some specific post offices, for the corresponding new bank notes. However, in order to do this, besides an identification document (e.g. passport), a "proof of address" is also required.
Obviously, a tourist, who is not resident in the UK does not have such document (unless the equivalent document in his home country is also accepted -- which I doubt; or maybe the hotel where he is staying could issue such certificate, because that's his "address" in the UK?)
TL;DR: Can a tourist go to one of the special post offices, and exchange old 20 and 50 Pound notes? Does he need a proof of address to do so? If yes, how can he get one?