I can't speak for UK folks, but in the US there are a limited amount of shops where a customer might negotiate a discount. At any brand-name retail store (such as Wal-Mart or Target) the prices are determined by a corporate office and employees have no authority to offer discounts. If, however, you are shopping at a privately owned shop that sells high-end appliances or furniture or jewelry, or at an automobile dealership, you might ask the salesperson "Is that your best price?" or "How much would [this item] cost if I paid you cash right now?". Or you could be coy and hover over a paricular item and when the salesperson asks "May I help you?" you could tell him/her "I really like this [item] but it's a bit out of my price range." Sometimes the salesperson will then begin to negotiate with you and offer a lower price.
If the shop assistant has no authority over the prices and has to tell a customer such, he or she would simply say something like "I'm sorry, but the prices are as marked. I can't offer any discounts."