Just fell down a rabbit hole and wound up reading Wikipedia on William Joyce, Lord Haw Haw
However, the Attorney General, Sir Hartley Shawcross, successfully argued that Joyce's possession of a British passport, even though he had misstated his nationality to get it, entitled him until it expired to British diplomatic protection in Germany and therefore he owed allegiance to the King at the time he began working for the Germans.
The historian A. J. P. Taylor remarked in his book English History 1914–1945 that "Technically, Joyce was hanged for making a false statement when applying for a passport, the usual penalty for which is a small fine."
thankfully for Wallace history is unlikely to repeat itself.