As we have a Constitutional Monarchy in the UK it’s Parliament, not the Royal Family, who is in charge of who is in line to the throne.
This was set down most importantly in the 1701 Act of Settlement which skipped over any number of people who would have been in the line as descendants of Charles I or James II, but were Catholic. This is how Protestant George the Elector of Hanover came to be first in the line of succession after his second cousin Queen Anne died without an heir.
This was amended in 2015 to mean that people in the line of succession who had married Catholics but did not themselves convert were reinstated (e.g. Prince Michael of Kent). However if you convert to Catholicism yourself, you’re still excluded (e.g. Lord Nicholas Windsor, the second son of the current Duke of Kent).
Given that the rules can and have been amended from time to time, I don’t think it would cause particular upheaval if Parliament were to consider that in order to be a serious potential candidate - in the top 10, for example - to be the Head of State you have to reside in the UK. I think most people would probably see that as fairly uncontroversial as it’s quite normal to have some kind of birth or residence restriction for high public offices in most countries. But I dont think it’s particularly likely here.