We are talking about the UK, so there is always to recourse to murder.
I sense a plot for an Agatha Christie novel here.
As Schnitte says, the Act of Parliament effecting Edward VIII’s abdication barred any heirs he might have from succeeding. But there was a minor quirk involved.
Under the 1772 Royal Marriages Act, descendants of George II were required to seek the monarch’s permission before marrying. The marriage would otherwise be invalid. Had that applied, George VI could have blocked any marriage the Duke of Windsor might have wished to enter into, whether with Mrs Simpson or anyone else. Except the 1936 Abdication Act specifically removed this requirement from the former Edward VIII and his heirs. Mainly because it was irrelevant, in that his heirs could never inherit the throne. But it also had the advantage that it avoided any need for George VI to authorise his brother’s marriage.
Just noting that the Royal Marriages Act 1772 could have some paradoxical and counterintuitive effects. Most notably, it provided that a marriage was void under British law if contracted without permission necessary under the Act. That sounds reasonable enough, until you realise that another piece of legislation (the Act of Settlement) disqualified someone from the line of succession if married to a Catholic. So the Royal Marriages Act actually offers a workaround for a member of the royal family who wants to marry a Catholic: Just do it without permission. That marriage would be null under British law, and hence not disqualify the royal. Children from that marriage would still be disqualified (because they’d be considered out of wedlock), but their parent would still be in as long as they don’t convert to Catholicism themselves, which is a separate ground for disqualification. This applies to George IV, who married a Catholic, remained in the line of succession because of this effect, and went on to marry a second time (Caroline of Brunswick) even though his first wife was still very much alive and considered her marriage valid under Canon law.
Roy Horniman beat her to it.
Horniman’s 1907 novel, Israel Rank: The Autobiography of a Criminal, was used as the basis of the screenplay of the highly-regarded 1949 black comedy Kind Hearts and Coronets and inspired the 2013 Broadway musical A Gentleman’s Guide to Love and Murder.