Loosehead, this is the relevant piece from Wikipedia; feel free to tell me if it's incorrect, but it seems to say we signed up to the final bit of the European convention as recently as 2003, long after the penalty had actually been removed for all crimes in this country.
The Criminal Damage Act 1971 abolished the offence of arson in royal dockyards.
The Naval Discipline Act 1957 reduced the scope of capital espionage from "all spies for the enemy" to spies on naval ships or bases.[5] Later the Armed Forces Act 1981 abolished the death penalty for espionage.[3] (In 1911 the Official Secrets Act had created another offence of espionage which carried a maximum sentence of fourteen years.)
Under a House of Lords amendment to the Crime and Disorder Act 1998 the death penalty was abolished for treason and piracy with violence, replacing it with life imprisonment (not mandatory).
On May 20, 1998, the House of Commons voted to ratify the 6th Protocol of the European Convention on Human Rights prohibiting capital punishment except "in time of war or imminent threat of war." The last remaining provisions for the death penalty under military jurisdiction (including in wartime) were removed when the Human Rights Act 1998 came into force on 9 November 1998. The UK later (10 October 2003) acceded to the 13th Protocol, which prohibits the death penalty under all circumstances.