They're talking about the monarch.
http://news.scotsman.com/scotland/Catholic-leaders-criticise-PM-for.6413415.jp
I don't think that religion matters that much. Whether you are Catholic or Muslim should not affect your ability or eligibility to be a prime minister
lol? So are you saying we should have a catholic head of state in a majority protestant country. It would be like having a protestant pope as head of the catholic church.