"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."
Don't need the whole murder, manslaughter thing. If anyone gets caught for this, it's committing a terrorist act they'll be done for. Attacking national infrastructure tends to get treated in that way.