The UK does not have a religious leader. Only England does because only England has a church established by law. All other denominations in England and elsewhere in the UK have their own religious leader, including Roman Catholics.
The monarch is head of the Church of England, she is also the queen of Great Britain, incidentally why is she referred to as Elizabeth 2nd when there has never been a 1st?
At the time of the accession of Edward VII it was decided that when the monarch had different regnal numbers in England and Scotland the higher of the two would be used. Thus if we ever have another King James he will be James VIII and not James III.
I don't think it was retrospective, jake, it's about the next one. (Given the apparent jump in numbers it would entail, I suspect there won't be a next one.)
If you really want to pick nits ... anyone, including Catholics, may aspire (in the sense of longing or yearning) to the position.
However since, as you point out indirectly, the position of the head of the C of E is hereditary, the number of people who are likely to attain that position is somewhat limited
There's lots of things you would have to do jake, re-apply for entry to the EEC, find somewhere to park your nuclear submarines (although we might let you keep them where they are - the rent wouldn't be too much) etc. There's lots more but I'm sure you can think of a few