There's a few over there that aren't on the map...I know that much. One near Milton Keynes that I know of for sure (we've chatted before). Where abouts in Gloucestershire are you located?
With fuel prices being what they are today, I would take that into consideration when making the purchase....it looks like a gasser, so expect something in the 4-to-5 miles/gallon range (assuming it's running properly, and driven gingerly). A full tank on an M35 (50 U.S. Gallons) would be about 190 liters, and at 104.5p/liter (current average petrol price0 that comes out to about £198 (or $387USD, about 2.25x the price here for a full tank). Let's just say my advise is to savor each and every mile driven at those prices! Forgive me for pointing out the obvious....I'd hate to see you get in to one of these and wind up in a bad spot.
On the bright side, parts are relatively cheap, and pretty easy to come by (same basic truck was made for 38 years or so). They are simple to work on (albeit large), and very well documented and understood by the MV community. If you ever need to know anything about these trucks, this is the place...bar none.
Anyways, it's great to have another Brit on the site. Cheers, and good luck!