Actually, a shop is under no obligation to sell you anything in the UK - having the item there is just an "invitation to treat" or something like that (IANAL) and they don't have to accept your offer to buy. From
wikipedia:
"In England and Wales, banknotes issued by the Bank of England are legal tender, meaning that they should be accepted in payment of a debt; they do not have to be accepted, but the debtor has a good defence in law against being sued for non-payment of that debt. In Scotland and Northern Ireland, no banknotes are legal tender, and each bank which issues banknotes does so in the form of its own promissory notes. In the Channel Islands and Isle of Man the local variations on the banknotes are legal tender in their respective jurisdiction.
Scottish, Northern Irish, Channel Islands and Manx notes are sometimes rejected by shops when used in England. British shopkeepers can choose to reject any payment, even if it would be legal tender in that jurisdiction, because no debt exists when the offer of payment is made at the same time as the offer of goods or services. When settling a restaurant bill after consuming the meal, or other debt the laws of legal tender do apply, but usually any reasonable method of settling the debt (such as credit card or cheque) will be accepted."