To be clear, I feel desperately sorry for his family and wish there was some way for them to get justice, but..
..there has never been the slightest chance of his alleged (accidental) killer being returned to the UK. No country would return the spouse of a member of their security services in these circumstances, but particularly not the US. It is therefore cruel and exploitative of the media and politicians to pretend to his family that 'raising the issue at high level' will make the slightest difference. It is wrong to give them false hope.