Today we learn that Downing Street and the Palace have discussed plans to change the rules of succession to the throne, removing the bar on marrying Roman Catholics and discretionary primogeniture. If this happens, and at the moment there is just a Private Members' Bill, then Princess Anne would leapfrog Princes Andrew and Edward (and their children) in line to the throne. Possibly not an idea she would relish. And, again getting ahead of ourselves, Kate Middleton could more easily convert to Catholicism if she wanted.
There are further complications. The Commonwealth needs to be consulted to ensure that they would be happy with any changes, or there could be a variety of heads of state (which might share the work around...) There may be some concern about the monarch's role as governor of the Church of England and her relationship with the Church of Scotland. The legislation is complicated, too: the law date from the Act of Settlement (1701), but have also been restated in other acts, such as the Act of Union (1707). I wonder what would happen if a case was brought under the Human Rights Act, too.
The government is clear that all this may take some time, and it points to some of the problems when two very different ideas of the constitution clash. In 1701, the monarch, the established church and, indeed, the nation, were seen as bound up, inseparable, and mutually supportive - a somewhat mythic, organic whole. Counter to this view might be a more contractual, rights-based society, based on consent and the principle of the separation of powers. The British story of liberty has always been about the tension between these two views of society, but the caution currently being shown in such matters suggests that the political balance may still lie with the former.
I wonder what Henry VIII would have made of either such caution (he'd have cut this gordian knot quickly, I imagine) and the legacy of the Reformation.