Off road tests were allowed some years ago, but now a H licence test must be conducted on the Queen's highway in and out of traffic-hence the legal requirements of registered,insured, etc etc. I think it is still allowed to have a 'commander' with you during the test if your vehicle has restricted views
it really is essential that you take your test in the type of vehicle you want to use.speeds and controls of tracked vehicles vary massively. I would not recommend taking your test in a tiny plastic 250kg 10mph argo, that has been converted into a 'tracked' vehicle, then driving around in a 50mph 8 ton cvrt or 12 ton 30mph 432-recipe for a nasty mishap.
this is one of very few countries where you can still drive armour on the road, and all it takes is for our bad news media to get hold of a story...........