My views:
1. The person concerned arguably made a mistake. It isn't good practice to leave a shotgun in plain sight (even if slipped, its obvious what it is), unattended, in public. To do so raises questions over whether you have met your safekeeping obligations. All would depend on the circumstances (e.g. location, time left etc) as to whether it was reasonable.
2. The police were heavy handed in revoking rather than having a stern word and reminding the holder of his obligations and the potential consequences. That would have been more proportionate, would probably have resulted in the holder thinking about it more carefully in future (benefiting public safety) and saved money and time on all sides.
3. But, the police would rather revoke and leave it to the courts to make the decision to return firearms than be blamed (again!) for doing so themselves. Doesn't matter the situation, they are institutionally terrified now of another Plymouth or John Lowe/puppy farm situation (plus all the others).
4. The court made the only logical decision. It also probably thought that the holder had been punished enough (and probably the police did too).
5. People fail to understand the "insurance" (aka membership) that they buy. Read what it actually covers rather than assuming that because BASC fought someone's case once, they will do it for you. BASC doesn't offer legal assistance insurance for revocations, but does sometimes (after a long winded process that involved Council approving it) back individual cases. Other policies (e.g. those mentioned above) specifically are for such situations.
6. Complacency does breed contempt. I know of people who have lost guns through such casual approaches to security. In once instance, a car was broken into during a farm burglary and a gun that had been left in there was stolen and never recovered. Probably wasn't the first time he'd left it there overnight.