FAQ for
Death at a Funeral (2007)

The death penalty is not legal in England, and as they did not kill Peter, the question is trivial

Further, since Peter was trying to blackmail them and the effort was rendered moot by the secret being revealed to all, Peter would have also been facing charges, not to mention the sheer number of witnesses against Peter's account of events and his case of being kidnapped and drugged would have been almost entirely his word against theirs. It would be interesting to know under British law who committed the worse offense though.