“Capital punishment still exists for certain offences. I think treason is one of them.”
Not so, 10CS. This sets out the position (which I have cribbed, so not all my own work):
“In 1965, the Murder (Abolition of Death Penalty) Act abolished capital punishment for all offences, except treason, piracy with violence and arson in Royal Dockyards, all of which remained capital crimes. This was confirmed in 1969, after a quinquennial review of the law.
The Human Rights Act 1998 incorporated the European Convention on Human Rights (ECHR) into UK law, banning capital punishment for murder except "in times of war or imminent threat of war". In January 1999, the then Home Secretary Jack Straw signed the Sixth Protocol of the ECHR, formally abolishing the death penalty in peacetime and in December that year, the Government ratified the Second Optional Protocol to the International Covenant on Civil and Political Rights (ICCPR). This was followed by ratification of Protocol 13 in 2002, thereby totally abolishing capital punishment in Britain, including during times of war.”
So, not available and never likely to be again.