OU studes
are eligible for the HE discount (don't know about the ADC Student Account, sorry). You can verify this if you have access to any higher ed networks: after clicking on Education Store -> Select your University, you'll see a pulldown menu asking for which region you're based in. Select (if memory serves...) the East option - you'll then notice the listing for the Open University. Select that, and you're looking at a top-end MB for £988.18, which is only £280 more than the top-end white MB cost in the HE store this time last week...
Due to the nature of the OU, most of its students won't be on its Milton Keynes campus, and unless they're working at another university, they'll probably not have access to a university network. If this is the case, you'll need to place your order over the phone. Remember to make sure the discount is applied to your order!