This comes up every so often the past several centuries.<g> Elizabeth II is the rightful Queen of the United Kingdom as she reigns by virtue of 6 Anne ch 7 as amended (by among other things, The Royal Marriages Act and the Abdication Act). If she couldn't base her claim upon act of Parliament, then the Duke of Bavaria, who is the successor to the House of Stuart, would probably be king. Or it could be one of the descendants of Hannah Lightfoot, or less likely, Maria FitzHerbert. The then future King George III married Hannah Lightfoot on 17 April 1759 and had three children by her. He married his queen, Charlotte of Mecklenburg-Strelitz in 1761. Hannah was living in 1768. And if you don't accept the evidence for Hannah Lightfoot, the future George IV as Prince of Wales, married against the provisions of the Royal Marriages Act and Act of Settlement on 15 Dec 1785 Maria FitzHerbert. I think they had a daughter. He married his queen in 1794 and hated her. He later went back to Maria.