Perhaps it ought to be debated, at least. I'm firmly anti, but I am uncomfortable with saying anything like this is now outside debate and need no longer be discussed.
I don't think it would pass the House of Commons even if it were not the case we'd have to completely change our relationship to Europe. Most MPs are apparently against it. As they are representatives, not delegates, they are not obliged to vote in accordance with constituents' wishes (whether they should is another whole topic). Opinion polls apparently suggest mopst people in the UK are pro-death penalty, though the number of pros falls significantly when the safeguards needed to ensure guilt are put to them.
I wonder if the pro-hanging camp would be prepared to vote for lavish legal aid and forensic services and automatic appeals so we could be as sure as possible we'd bumped off the guilty, not the innocent? The US experience is a salutary one. It seems even very traditionally pro-death penalty states are finding approval rates falling as various initiatives (like the Innocence Project) expose the large number of unsafe capital convictions.