Posting this for the off-chance that someone else is in my shoes (US Forces serving in the UK)
Get a copy of "Declaration of personal goods imported by member of visiting forces C2". Fill it in, sign it and send it to Vans to finish off the top portion. Have them include it in the shipment and you will not have to pay UK VAT on the items. You might have to make a phone call to the port to explain the situation, but I just accepted delivery of my empanage and did not have to pay UK taxes or duties!
Please feel free to contact me if you have any questions.
RV-7: Empannage complet, working on aft fuselage