What is High Treason In The United Kingdom?

Legal Definition
Under the law of the United Kingdom, high treason is the crime of disloyalty to the Crown. Offences constituting high treason include plotting the murder of the sovereign; committing adultery with the sovereign's consort, with the sovereign's eldest unmarried daughter, or with the wife of the heir to the throne; levying war against the sovereign and adhering to the sovereign's enemies, giving them aid or comfort; and attempting to undermine the lawfully established line of succession. Several other crimes have historically been categorised as high treason, including counterfeiting money and being a Catholic priest.

High treason was formerly distinguished from petty treason, a treason committed against a subject of the sovereign, the scope of which was limited by statute to the murder of a legal superior. Petty treason comprised the murder of a master by his servant, of a husband by his wife, or of a bishop. Petty treason ceased to be a distinct offence from murder in 1828, and consequently high treason is today often referred to simply as treason.

Considered to be the most serious of offences, high treason was often met with extraordinary punishment, because it threatened the security of the state. Hanging, drawing and quartering was often employed. The last treason trial was that of William Joyce, "Lord Haw-Haw", who was executed by hanging in 1946.

Since the Crime and Disorder Act 1998 became law, the maximum sentence for treason in the UK has been life imprisonment.
-- Wikipedia