Royal weddings bring with them fanfares, fascinators, and fol-de-rol (folie de roi?) — and a renewed interest in the singular business of constituting and propagating a monarchy. A former family law teacher, I've a modest interest in the marriage, as opposed to wedding, side of things, because while, as everyone knows, the rich are different from you and me, the royal family is even less like us when it comes to tying the knot.
Some things royal are governed by tradition in its guise as the common law. Succession to the throne, for instance, operates in part [see also the Act of Settlement (1700)] according to the law of (male) primogeniture insofar as it passes that "estate" to the first-born son in preference to daughters or younger siblings. (There have been recent attempts to alter this by statute to remove the sexism. Interestingly, some believe that any such change would have to get the approval of other nations for whom the Queen is head of state, such as Canada, or would at least require consultation with such states.)
Other matters, marriage matters, are governed by explicit legislation. Two hurdles face any would-be royal bride or groom: the ruling monarch must give consent, and in any event neither party may be a Roman Catholic.
The latter restriction — clearly at odds with modern human rights legislation, as is the succession law favouring males — is found in the Act of Settlement (1700):
And it was thereby further enacted That all and every Person and Persons that then were or afterwards should be reconciled to or shall hold Communion with the See or Church of Rome or should professe the Popish Religion or marry a Papist should be excluded and are by that Act made for ever incapable to inherit possess or enjoy the Crown…
(Plain language fans and drafts-people feast your eyes on this legislation: A sentence of 1,109 words from the opening capital letter to the one and only full stop.)
The requirement of royal consent was instituted in the Royal Marriages Act 1772, when, so it is said, George III felt his brothers had married unsuitable partners and proposed that thereafter the monarch's consent ought to be required. The core of that statute is included in the Queen's instrument of consent, pictured below, permitting William to marry Kate tomorrow.
There is an escape clause in the act, however, giving a royal over the age of 25 the right to contract a valid marriage with whomever, provided notice is given to the Privy Council of the intention to marry despite refused consent and provided that both Houses of Parliament do not "expressly declare their disapprobation of such intended marriage." There's some lack of clarity, I think, as to whether the marriage would be good if contracted before Parliament has its say — but that's for another wedding day, perhaps.