Well how do you intend to stop him. Arrest him when he lands at a UK airport under some arcane law ? Maybe they could use this still extant act from 1351 and have him arrested by Beefeaters and held in the Tower.
adhered to the king's enemies in his realm, giving them aid and comfort in his realm or elsewhere;
violated the king's companion,
or this catch all would probably do the job !
The act originally envisaged that further forms of treason would arise that would not be covered by the act, so it legislated for this possibility. The words from "et pr ceo q plusurs auts cases de semblable treson" onwards have been translated as:
And because that many other like Cases of Treason may happen in Time to come, which a Man cannot think nor declare at this present Time; it is accorded, That if any other Case, supposed Treason, which is not above specified, doth happen before any Justices, the Justices shall tarry without any going to Judgement of the Treason till the Cause be shewed and declared before the King and his Parliament, whether it ought to be judged Treason or other Felony.
en.wikipedia.org/wiki/Treason_Act_1351