Two points, TOFO.
First, the only person in those circumstances that could have changed what came next was the arresting officer. Show me who else?
Second, had he *the Police officer in question) been 'sensible' about it, I doubt he would have faced disciplinary action, under the circumstances as his actions could be easily justified - by common sense, if nothing else.
That was the only point in the chain of events where any flexibility existed.
"Thank you, Sir, for being so honest and public spirited. However, I have to point out to you that you may have committed an offence under the Firearms Act 1968 in that I believe that you were in possession of an unlinceced firearm. I have to point out that to do so is a breech of the law. In veiw of the fact that you appear to have acted in the public interest without intention to commit further crime, I must caution you that any future action of this nature on your behalf will be treated as a serious crime and you may be procecuted."
I am, of course now guilty of serious thread drift as I am refering to the second case cited here, not that raised by the OP.