Apple has stores in the Westfield in Straford and Shepards Bush. Your shopping experience there will be much better than at the Regent Store which likely has endless queues and tourists in there.
Flagship Stores always have huge lines. But, they do take that in consideration and have straight stock.
Try it, then if not.. Try another store in London..
Or try a Tesco, Currys, Carphone Warehouse, Phones4U.