If push does come to shove, it's for the State to prove that you committed an offence by importation, manufacture or modification, not for you to disprove it. Only after they've made their case would you need to provide a defence.
I'm very much not inciting you to die on this hill, and I'd be OK with completing a brief declaration to end the matter. Just noting that if you do get a knock on the door, you can be friendly, honest and cooperative - you're an active airsofter and can demonstrate it - but either way the less you say, the better. I wouldn't volunteer any information about where your collection came from.
I'd be more concerned about the prospect of being billed for the VAT. If HMRC come after you directly, I'd pay up, as they're a big enough gang to wring it out of you. If UPS come after you saying that you owe it to them, I'd simply ignore them - you have no contract with them (except implicitly by performance), and agreed to no such thing.
Either way, thanks for keeping us posted on the latest plot twists and characters in this shit-show.