According to Wikipedia -( I know not the best source)
High treason today comprises only:
Treason Act 1351:
encompassing the Sovereign's death;
killing or conspiring or attempting to kill the Sovereign's wife or eldest son;
violating the Sovereign's wife, or the Sovereign's eldest unmarried daughter, or the Sovereign's eldest son's wife;
levying war against the Sovereign in the United Kingdom;
adhering to the Sovereign's enemies, giving them aid and comfort, in the realm or elsewhere;
killing the King's Chancellor, Treasurer (an office long in commission) or Justices; and
Treason Act 1702:
attempting to hinder the succession under the Bill of Rights 1689 and the Act of Settlement 1701.
Note the one about violating the wife of the sovreigns eldest son.
The mirror tried to use this to get James Hewitt prosecuted for treason for having an affair with Princess Di
It seems that sleeping with neurotic princesses is as vile a crime as conspiring against your country