As Chakka says, it depends on the Cathedral. This from the St Paul's website.
Marriages according to the Rites of the Church of England take place in the Chapel of The Order of the British Empire located in the crypt of St Paul�s, subject to a successful application for a Special Licence granted by the Archbishop of Canterbury. This privilege is extended only to members of the Order of St Michael and St George, the Order of the British Empire, holders of the British Empire Medal, and their children (but not grandchildren).