No matter what we may think about the rights and wrongs, the moral aspects or the record or intentions of those involved, the issue is (sadly, I agree) very simple. UK firearms law (nor most other laws) makes little or no allowance for circumstances. In fact that law is one of the clearest we have. If you are in possession of a firearm for which you do not hold a licence, you are committing a crime for which there is a clearly laid-down penalty in civil law.
OmegaV6, you cite a case where the outcome seems so appallingly unfair. The point is that he did, clearly commit a crime under the act and the jury would have had no option but to find him guilty. The judge would have been bound by sentencing guidelines. The only bloke that could have made a difference to the whole issue (once the poor lad had taken possession of the shotgun) was the arresting officer. Maybe he could have handled things differently. Once arrested, the guy's fate was sealed. Sad, but true. IMHO, the guy acted very honourably and went to prison for so doing.
Now, had I been in Paul Clarke's position, even though I know the Act, I would probably have done the same thing, expecting better treatment from the Police for, potentially, stopping very nasty crime or even helping to solve one. As for the SAS gent, I have been told EVERY time I left theatre not to take any hardware with me as souvenirs. Different cases, same law, I'm afraid.