I'm very sorry to hear this.
We've had a few members who've had Dibble show up to look into their importation of an assault-style-toy. However, nobody's been prosecuted or convicted for it (absent some chancers with CO2 powered 'self defence' .50 cal paintball guns).
So, deep breaths. It's a long, lonnnng way from here to any real consequences.
What I'd suggest that you do is to contact
https://www.ukapu.org.uk/ and ask for advice. Someone there will be able to talk you through the legislation, the airsoft defence that applies, and how you can demonstrate it, should you need to.
If it were me, I'd print off several copies of an information pack listing the offence (VCRA 2006, S36 causing a realistic imitation firearm to be imported), the defence (S37 as amended, permitted activity, airsoft use) and your defence (membership of an airsoft skirmish site with 3rd party liability insurance, I hope), plus contact details for anyone who can confirm that. Keep this handy, get in some decent tea and a nice biscuit selection (Garibaldis!), and welcome in any coppers sent out to investigate your criminal empire - they won't have any real clue why they've been sent out.
It almost certainly won't come to that, but better to have the information and not need it, than to need it and not have it.