I have long been interested in fifth wheels for defenders, try 'vipex.co.uk' for some seriously expensive food for thought. (no joy with the link function today!)
Try a search for 'land rover fifth wheel', 'articulated land rover', and 'mini artic' (exclude arctic), there have been lots over the years, generally the trailer stays 3.5t and the tow vehicle bears the extra payload on the fifth wheel or goose neck; although some Iveco combos I have seen up to 12t! The Fifth Wheel Company in Wales do mainly caravans but have done a few trailers. I think the main reason it is not popular in the UK is precisely because it is a bit of a grey area. The companies that do it serve a niche market and charge royally for it, but one day I'll do my own