You will struggle to get an off road tyre for a big wheel, better to go for a 15" or 16" Wheel with a big tyre. I like BFG Muds which I have on the defender however insa turbos will be cheaper.
I'm struggling to find an all terrain tyre for my 18" disco rims.
as jono said, smaller wheels and big tyres give you load more options and are loads cheaper, insa turbos good if you want cheap!!
BFG have launched an 18" AT but I think it's eye wateringly expensive.
A set of 16" steel wheels are ideal. You don't want alloys offroad and the higher the sidewall the better so no 18s!!!
I used to have 235/85/16's on my series 3, the tyres raised the Land Rover by an inch. The Insa turbos I had were noisey though.
Best tyres so far were the Greenway Anaconda, think Colway made them and they've gone bust now!