It’s technically possible ( certainly more so than the fixed PSTN switching network ) but is it desirable ?,
The original PSTN linked numbering scheme ( going back decades ) was useful, national calls cost more than local ones so having a different STD code for most large towns and citys enabled customers to know the likely scale of charges , and in a practical sense , ( for example ) if you wanted a local plumber you wouldn’t pick one with an STD code of a town 50 miles away, also the police ( for one ) are keen to keep a relationship between someone’s landline phone number and their geographical location, obviously a regular call to the police ( not 999 ) with a STD code CLI indicating the town the call was made from is ‘useful’ .
I would imagine the same constraints as far as taking a phone number with you to a new address will remain in place ( as far as moving a phone number to a different geographical exchange area ) , even if there are effectively no ‘exchanges’ once the PSTN network is retired and everything is IP .
It would be simple to have any number anywhere , ultimately though , BT don’t own their phone number ranges , ( no provider does ) they are allocated by Ofcom , who will be the ultimate decider if the linked number scheme remains or can be allowed to drift.
