I think the Pricing Order 2004 applies whether there is a contract or not:

"Obligation to indicate selling price

4.—(1) Subject to paragraph (2) and articles 9 and 10, where a trader indicates that any product is or may be for sale to a consumer, he shall indicate the selling price of that product in accordance with the provisions of this Order.

(2) The requirement in paragraph (1) above shall not apply in respect of:

(a)products sold from bulk; or
(b)an advertisement for a product."

(Articles 9 & 10 refer to general reductions (eg 10% off store-wide) and precious metals where the price varies with rise and fall of the market price of the metal)

LFF