hey, i had this same problem. As art courses start on the first or so. I sent an enquiry to some accoms a while ago and I got told contracts are fixed. There is one accom (New wharf hall) which offers a contract earlier for Art students I think but I still chose a different one as I am picky and can travel in for a while. Hope this help a lil
I'm sure if you contact your accommodation they will let you move in early.
Grosvenor House's move-in date is the 19th September, however - if a resident informed us that their course began earlier, then we would do our best to accommodate them sooner.