Wonder if it would be possible just put a 'lift kit' of sorts and maybe some bigger wheels/tyres on a regular caravan? Might be an idea
Aussies have already done that! You would be amazed where they take them!
As for off road conversions, there are plenty of outfits in the UK that convert and sell expedition vehicles. Most of these either are roof tents or internal rear seat bed based, which can be cramped.
You could find an old dormobile, or convert a f/c - there was a nice one in a mag not so long ago.
If you leave the LR scene, there are many jap 4x4 campers such as toyota lite ace, hi ace etc or for more serious stuff, the landrcruiser comes with a body, nissan patrol do a hi-top internal conversion where everything disappears into the side panels.
Truck demountables are probably the cheapest UK option, that will fit on a pick-up.
Depends what you are looking for really. Self build would give you what you want for less money - expedition vehicles do get their price!
Couple of links to aussie off road caravans - you can get the forest river brand over here which essentially is very similar, although a US model.
http://www.bushtracker.com/Photo.htm
http://www.trakmaster.com.au/
