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.
On the one hand is attempted blackmail. On the other hand is forcible restraint, physical assault, the intentional administering of a noxious or controlled substance, negligence in properly administering it, protecting a drug dealer, and failing to render aid to the victim which could reasonably be called attempted murder given the motives of avoiding blackmail and prosecution on drug charges. Of course, he may well have died from the overdose, making it second-degree murder, but the movie ended before we found out. Either way, it would be a stretch to say that everyone at the funeral would claim that the guy came to the funeral in perfect condition, and then somehow tied himself up and put himself in a casket further off the floor than he can see, and overdosed himself with exactly the same drugs as their own fool "pharmacist" was making.