Not too au fait with the AT courses, but I do know a bit about the Island. Everything normally stands as would any other barracks, really- Cookhouse is PAYD, Spar shop is overpriced. If you can, bring your own transport- There isn't an awful lot nearby for nights out otherwise, although a taxi to the train station costs around Â£5, and a return to Portsmouth harbour is less than that. I would imagine you would be in the transit accom, and for transit accom it really isn't too bad, to be honest.
For detailed info on the actual AT course, I'd get in touch with whoever put you on it and ask for a phone number.
If that is no joy, try getting hold of the Hive on the island, they may be able to help out a bit.