Hiya Dave,
I don't have any advice on the actual importing of US spec vehicles to the UK, but I can offer some observations on the vehicles available in this market.
A lot of the potential busbuyers in the States pay attention to vehicles being offered for sale in the southwest (Arizona, southern California, New Mexico) because of the arid nature of this area. No moisture - very little rust. The flip side of this coin is that you can usually expect all the rubber (seals and the like) to be thoroughly dried out and interior vinyl in pretty bad shape.
The actual numbers of busses vary around the country also. The further east one goes the fewer busses (or any older vehicle for that matter) one will find. This again is due to rust. The higher humidity in the east coupled with a penchant for salting the roads during the winter tends to limit the lifespan of a vehicle.
I live in the Pacific Northwest and while it tends to be "moist" here, there are a great many busses to be found in this area. I have six of them within a few blocks of my house. You might want to look through the Craigslist listings for Portland (http://portland.craigslist.org/) or Seattle (http://seattle.craigslist.org/) and check out the busses for sale. Often they can be had for around the $1000 mark. However, since you have to MOT your vehicles I'm thinking that you might not want to shop the bottom end of the prices. Even busses advertised as being in good shape mechanically here in the States will likely not come close to passing your MOT.
Let me know if there's anything I might be able to do to help out.