Not totally, Jan.  If the monarch is to remain the head of the Church of England (and I'm not getting into that particular debate here), he or she could NOT be a practising Roman Catholic.  That particular rule was introduced for a very good reason; the ensuring that no future monarch could pull England (as it was at the time) back into the arms of the Papacy.

I'd agree that this was probably a heavy-handed approach, but that was the underlying reason.

It is outdated now though for this day and age (IMO) as faith/belief should be a personal thing, (not something one is forced to believe) - with no discrimination, as there should not be discrimination over gender.