This is the UK. We don't have the construct of first degree murder. I feel it might be quite challenging to prove that $whatever was released specifically to kill, which is what you'd need for a pre-meditated murder conviction (UK's equivalent of first degree) but causing death by being a silly bugger (AKA manslaughter) would be more likely to succeed.
Nope, I'm not a lawyer nor do I work for the police. I just work in IT so take an interest for...ummm...idle curiosity. Yes, that's it. Definitely that.