As other have indicated, a trader is entitled to refuse any type of payment (unless, in some bizarre way, he manages to discriminate against them on the grounds of their race, gender, religion, etc, by doing so).
The term 'legal tender' only comes into play when, for example, someone is trying to sue another for non-payment of debt. If the debtor has offered 'legal tender' as a form of payment (e.g. banknotes rather than bags of jelly babies), he/she can't be sued if the other person has then declined to accept it.
in England and Wales, 'legal tender' means Bank of England banknotes and Royal Mint coins. (However you can't use 1p coins as legal tender to pay your Council Tax bill, as 1p and 2p coins are only legal tender for debts of up to 20p).
In Scotland and Northern Ireland, somewhat oddly, NO banknotes are 'legal tender'. Only Royal Mint coins are.
https://www.bankofengland.co.uk/knowledgebank/what-is-legal-tender