Not on O2.
My Mum qualified for her upgrade on Wednesday past, and every O2 store in Glasgow has no 16 or 32GB iPhone 4S in stock. Some have 64, but not many.
The stores have no idea when stock is back in, they don't get told by Head Office when they're getting stock or how much, and to add insult to injury they're running an offer for existing customers where the 16GB is free to upgrading customers on the £41 per month tariff (normally £49.99 for the phone).
Thing is right, it runs until November 30th, but Glasgow stores are quoting up to another two weeks without stock. So the offer could end before the stock comes in or you manage to find a store with stock. My Mum ended up going for an iPhone 4 8GB and loves it.