I don’t think there’s anything in the Constitution to prevent it, but as I understand it, royal marriages require permission from the monarch. Importantly, the monarch is the head of the Church of England and as such, has a duty to uphold its tenets which don’t include acceptance of marriage to a divorcee whose ex-spouse is still living. That is why, although the Queen gave her permission for Charles and Camilla to be married, they were unable to marry in church. There really is nothing to stop Camilla becoming queen.