You can choose whatever suppliers you want as long as it's within the agreed money. I have bought everything myself from the Internet and have sent off all my receipts to the DSA team about a week ago, should get my money back by the end of next week. By doing this, I have all the upgrade I wanted and would have wanted in an ideal world and still managed to save them around £150. It's a bit annoying to do so, but it's worth it. Some suppliers would be willing to price match, like my uni has a supplier they work with regularly hence they were wiling to price match the entire list from the agreed supplier, but still they wanted too much for the upgrade I wanted; they wanted £300 for the upgrade on my Mac, whereas if I went for Apple directly, it costs the same as the lower specs quoted from the supplier.
here is a a number you might want to try, but you can only use it once as it's the secret number only universities should call 01325 215194. Just tell them your uni gave it to you and they will answer your questions, but probably only once. Otherwise, when you email them, ask for a direct phone number. My personal experience with the DSA team has been very positive, but yes the actual SFE number and the people are useless. It told the guy 20 mins to have found out he doesn't actually know the answer to my question.