That’s what Sweden has been doing yes. However it very much is a “mutual understanding” type situation where Sweden is very attached to the SKE and the EU just doesn’t really want to bother due to the situation being low-stakes high-friction. The UK also fits all criteria for “must use the Euro”, but would not benefit from the same “mutual understanding” that Sweden does because the stakes for the Euro would actually be very large. It is quite unlikely that the EU would just bend the rules and let the UK keep their own currency this time. It is also quite unlikely that the British public would even come close to accepting the Euro.

Regardless of your position on the matter, remember that Brexit negotiations completely broke down multiple times over much, much simpler and lower-stakes questions. The British Ego is at least as large as the French’s, and if four years of Brexit should have taught us anything it’s that they have extremely unrealistic expectations and actually think that the EU should bend over backwards to have them back.