This is a common misconception around the definition of legal tender.
A shop is perfectly within their rights to refuse to sell you something if you are not willing to use their accepted payment methods.
Legal tender simply means they can't sue you if you offer to settle their debt using a legal tender currency. If they have refused to sell you the item, there is no debt, therefore no obligation to accept any particular instrument
https://www.bankofengland.co.uk/explainers/what-is-legal-ten...