In a sane society, this wouldn’t even be an issue. But, we’re talking about an insane government that took away just about everybody’s guns...
Actually very few guns were 'taken away'. Legally owned guns in Britain have always been predominantly shotguns and various categories of hunting rifle: and these have never been banned. The number of handguns etc was always very small in comparison, for all sorts of historical reasons. Given the increasing popularity of countryside shooting sports in Britain, it's quite possible indeed that there are now more legally-owned guns in Britain than at any time in the past.
As for this case, and others like it, if the police are called to the scene of a shooting in which B appears to have been shot by A, it's not unreasonable of them not immediately to accept A's version of events, however plausible it may seem, without thoroughly investigating the facts. Indeed it would be quite unprofessional of the police not to do so. As it turns out A's account appears to have been verified, and they will not face charges. Difficult to see anything wrong with this. This has been the procedure in a number of similar cases.