Basically the death penalty cannot be re-introduced to the UK without the UK leaving the European Union.
I'll leave you to 'google' for the relevant documents, but Protocol 6 of the European Convention on Human Rights (ratified and incorporated into UK law in November 1999) bans the use of the death penalty in times of peace, and Protocol 13 (ratified etc. July 2003) is an absolute ban on its use.
Furthermore it is now against EU law to extradite suspects from any EU country to another country where they may suffer the death penalty upon conviction of their crime.
I'll leave you to 'google' for the relevant documents, but Protocol 6 of the European Convention on Human Rights (ratified and incorporated into UK law in November 1999) bans the use of the death penalty in times of peace, and Protocol 13 (ratified etc. July 2003) is an absolute ban on its use.
Furthermore it is now against EU law to extradite suspects from any EU country to another country where they may suffer the death penalty upon conviction of their crime.