Some thoughts for Gordon Brown:
Allowing a Catholic to sit upon the throne would put him in a terrible conflict of conscience. As a Catholic he would be in full communion with the Holy Father in Rome; as sovereign his coronation oath would oblige him to keep many Anglicans out of full communion with the same Catholic Church. Regardless of the laws of Westminster, the laws of conscience would preclude a Catholic from serving as head of the Church of England. Again, disestablishment must come first. But as I oppose disestablishment as an unwelcome step toward secularism, the Catholic question should not arise at all.The Young Fogey himself, Rafal Heydel Mankoo, made some more practical objections back in 2007:
The requirement for unanimity brings with it other perils. Requiring all Commonwealth Realms to consent will inevitably lead to a debate within each realm as to the continuing relevance of the Monarchy itself. Governments of nations with strong republican elements will no doubt face a question of this sort: As we are examining the succession to the position of head of state surely this is the time to embark upon whole-scale reform.
Those who call for change should realise that any attempt to alter the Act of Settlement will stir a hornet's nest in various Commonwealth Realms which may ultimately result in the transformation of many from constitutional monarchy to republic. Of course the counter argument is that it is better to deal with the issue now, during the stable era of The Queen's reign, rather than to wait until forced to deal with it in an uncertain future.