Well, you will have to pick and choose your campsite thats for sure! Thats why a couple of people have produced a guide called the Big Pitch Guide (www.the-big-pitch-guide.com
) which details all the campsites in the UK which can take 40' A class and 5th wheel campers. Also they have good details showing the entrance roads and what to look out for etc on the way in.
Some of the smaller roads will definately be out of bounds I think, like some of those winding narrow lanes in Cornwall or the Lake District. Especially with stone walls either side...). BUT, if a tanker truck can get down there to deliver supplies, fuel etc then I dont see why a 5th wheel couldnt do it. Will let you know when Ive tried it...
There is a company here manufacturing 5th wheel campers now, the Fifth Wheel Company. They make units specifically for the Japanese type of pickup (Nissan Navara, Toyota Hilux, etc) which are more popular here. They are a bit shorter (27' overall length) but are still quite heavy (7000 lbs dry weight). They look nice but were too expensive for us which is why we went the US route. Ive attached a picture of their Celtic Rambler unit.
I think we made the right choice, Ive always had US vehicles so driving the Silverado is no issue for me (we have a 2003 2500HD with the 6.0 gasoline engine and LPG conversion). Our plan is to do some UK only trips next year with a view to going into Europe the following years, maybe as far as Spain. Longer term I plan to follow the European Drag Racing Championship one year when my son is old enough to appreciate the road trip!
Ive attached a picture of my Sabre too, but Im sure you know what it looks like already..