I am not convinced that a contract exists until all terms are agreed, including the method of payment. However, even taking what you say to be correct, I do not see that the seller has entered into a contract to accept cash or not to reject any particular form of cash. There are notices at the tills showing what credit and debit cards are accepted, which I am sure do constitute an offer to accept those as a form of payment, but I cannot recall any notices at B&Q stating that they will accept payment by cash.
Colin Bignell