This issue with that is Wales and Northern Ireland haven’t been independent territories either.
England conquered them. They haven’t voluntarily joined a union, they have been conquered.
Northern Ireland with “power sharing” meaning they cannot elect a democratic parliament is essentially is run as a colony. The only caveat being they do have seats in the UK parliament.
Wales is a semi-autonomous part of England with a local government having some say but no ultimate control should the national government decide against something. Again they have seats in the national parliament so they aren’t a colony.
Essentially in any other place Wales would be just part of England, not a separate country. Not a separate territory as there’s no significance to the border except a historical one.
Wales being a separate country is debatable.
They went from being a principality with some sovereignty to having none.
Currently they have devolved powered but the UK parliament has full sovereignty and can veto anything the Senedd decides.
They have no currency or mint. No separate legal system. No separate military.
Essentially they are were a part of England on joining the UK and their sovereignty comes from the UK parliament.
If Scotland left and the Union was broken they’d be a part of England again.
Northern Ireland is complicated.