Britain would have to leave the ECHR which it is highly unlikely to do.
I am strongly opposed to the death penalty because I believe:
it is an easy way out for the criminal
it is wrong to make good people kill a person
it can place a huge burden of guilt on the victim and/or the victim's family
the death penalty is hugely expensive compared to life imprisonment, using the US model as the example
Some abhorrent murderers have gone on to lead blame free, productive lives after serving their sentence, such as child murderer (in both senses) Mary Bell and a wife murderer who went on to become my barber. Of course some murderers should never be released.