Why dont we hang repeat offenders in the uk?

Right simple question. When we have a repeat offender here in the uk who has been caught he as obviously demonstarted that he cannot change, therefore should not be released, so why dont we hang them and save money keeping them locked uo? Im looking for your opinions on this one as I beleive that the majority of the public (and we do supposedly live in a democracy) would like this or at least be ok with this happening. Please let me know your thoughts to gage public opinion?