its only channel islanders, naturalised immigrants and manx who are allowed to choose which UK constituent they represent without restriction
There is also a loophole in which British people born abroad are eligible to play for any of the four UK nations. This is how Maik Taylor, born in Germany to a German mother and English father, plays for Northern Ireland despite previously having no ties to it whatsoever.
Not that this applies to Gareth Bale of course, but it's an interesting one.