What is the most serious crime in British law?

Hope this is right : high treason

Explanation:

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 ...

Murder I think big that's a guess do don't go with it