Hi I'm in the wilds of Cumbria already ahead of you in Airstream acquisition.
A few thoughts. Capture one in the States and you'll need pay for roll-on roll-off transportation, agents at both ends, and swinging UK taxes (10% + 17.5% VAT) of purchase price plus transportation costs on arrival.
Within your planning you need to be aware your Airstream will have electric brakes which need a special kit fitting to your car to work, and also you'll need a 2" (as opposed to a 50mm European standard) tow-ball to pull it (I can help in this respect).
Also your van will be wired probably for 12v
and 120v electrics, so you'll need to re-wire for 240volts.
Just thought you'd like to know. I myself am working on restoration having just got the chassis and floor finished. Wiring and plumbing next.....
If you go to the Hershy show in Pennsylvania in October (its held in the grounds of the chocolate factory) you'll very probably capture an Airstream there. But don't buy a big one or you'll end up needing a 7 ton truck in the UK to tow it legally! Best of luck