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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.
It is 'treason felony' to "compass, imagine, invent, devise, or intend":
to deprive the sovereign of the Crown,
to levy war against the sovereign "in order by force or constraint to compel her to change her measures or counsels, or in order to put any force or constraint upon or in order to intimidate or overawe both Houses or either House of Parliament", or
to "move or stir" any foreigner to invade the United Kingdom or any other country belonging to the sovereign.
The essence of all these involve offences against the person of the sovereign or her right to sit upon the throne.
Nothing there about giving away 'our sovereignty' to another state (it's the monarch's sovereignty, after all, not the subject's; the monarch is free to do what they want with their sovereignty - including sharing it if they so choose.).