Just looking at it from a law perspective, if there's a reason, (could be any of many, used in crime, threatened someone with it, no certificate amongst a rake of others) any police officer can seize your firearms. In practice, for planned operations AFOs will be part of it, who will (hopefully) know how to make firearms safe. But in, for example, a developing situation or other unplanned incident, that might not be immediately possible. In rural parts especially, the nearest AFO might be many miles away, be engaged on something else and possibly cannot attend for hours.
Anyway, if a police officer is seizing your firearm and you stick the finger up at him saying that he can't because there's no AFO there, do let me know how that works out. But don't waste your PACE rights when you land in the custody office doing it!
It's true that many police officers are pretty clueless about handling firearms, despite most of 'em getting some input during their basic training. But the thing is, police officers every day will be coming across incidents that they have not been trained for. It's the nature of the job. It is simply not possible to teach every law in the land to every police officer or to train them for every scenario they might encounter. But as a society we must rely that they will act in a reasonable and safe manner. And although many of us will be able to tell of stories they have heard or witnessed where that hasn't happened, mostly they do.