Been doing a bit of digging and this is what I've come up with so far (I may be wrong of course)
All this relates to Orders Totalling above £135 (Since Gunfire in particular require this for an order)
You can look up the Trade Tariff Commodity Code relating to Airsoft here:
https://www.gov.uk/trade-tariff
The code I found most relevant was 9304 (Other arms (for example, spring, air or gas guns and pistols, truncheons))...
This category brings up this info
Consignments valued at more than £135
Normal VAT and customs rules will apply on importation of the goods into Great Britain from outside the UK
From what I read HERE, the VAT can be applied at source by the seller in the originating country if they're registered to do so (which most European suppliers already are)
Or they may charge local TAX (which is higher than UK) (I don't think this is common for Airsoft though)
If not included in the sale then it will be applied by Customs upon entering the UK and passed on to the post service who will ask you for payment before delivering (plus a handling fee because they have to pay Customs)
So the VAT situation doesn't seem to have altered as most retailers add VAT to UK orders anyway.
Going by the info above, the new fee is the Duty at 2% of the order value. (Not sure if that includes delivery costs)
I've emailed Gunfire for their calculation of fees but not holding my breath for a clear answer.
....its a bloody mess...