I don't know how things work in Great Britain, but...
I'd assume if Environmental Health got invovled, they'd have to contact the police to get into the person's flat. I don't know if they can open other people's mail, but neighbors certainly aren't supposed to.

Shipping perishable food products such as stinky-melon or Sardinian maggot cheese from one country to another without going through customs isn't supposed to happen.

I'd be more concerned about whether the neighbor is OK. I also hope we learn how this turns out.