Guys,
I took my H test some years ago as a civvie in a FV434.
Basically you apply for a test in the normal way, stating the class of vehicle you wish to be tested on. Now not all test stations have an approved tester, so best to check with your local office.
The test was a "normal" test, turns, 3 pointer, emergency stop etc.
Just on the road in road legal tracked vehicle!
(A bv206 type won't do, as its steered by articulation).
Check the MV mags though, I am sure some enterprising type is offering a driving course and test.
David