I used to go off roading with my old man in his Land Rovers, the best thing would be to buy a copy of Land Rover Owner magazine as they always advertise off road days / excursions in them, that way when you go to them you will meet fellow off road nuts who will tell you the best places to go.

My favourite so far has to be Bala in Wales, it has tracks from "novice" right through to "I couldn't give a shit about my car". Trentham Gardens is usually good too but its a busy site and not that big.