There certainly is law on the question. The following is the present position : �1 and �2 coin to any amount; 20p and 50p to �10; 10p and 5p to �5; 2p and 1p to 20p only.
It is certainly not the law that payment in legal tender must be accepted by a vendor. Someone selling can insist on being paid in any currency even foreign or in goods or services or in the waiving of an existing debt or however the 'consideration' to use the legal term is demanded. Until payment is agreed there is no transfer of title in the goods anyway. The 'copper' cited must have been talking through his helmet; anyway he was wrong. However a creditor is not allowed by law to refuse the debtor's payment of the debt in legal tender, that is in coin or notes to the limits given here..