Hi Simon. Welcome!! There are a lot of Airstreams for sale on the forums here under the classifieds. Also some people list there Airstreams on a thread on the main forum. Since your on the UK, there is an airstream group from UK that I joined. Here is their link:
http://europe.airstream.com.
Also if you just search under Airstream UK, you'll see a number of sites for dealers and private parties who are selling new or used Airstreams.
Good luck and have fun on this new journey!!