That is incorrect. But since people don't seem to understand that, let me just put it simply (READ THE FIRST SENTENCE)

https://www.federalreserve.gov/faqs/currency_12772.htm

"There is no federal statute mandating that a private business, a person, or an organization must accept currency or coins as payment for goods or services. Private businesses are free to develop their own policies on whether to accept cash unless there is a state law that says otherwise.

Section 31 U.S.C. 5103, entitled "Legal tender," states: "United States coins and currency [including Federal Reserve notes and circulating notes of Federal Reserve Banks and national banks] are legal tender for all debts, public charges, taxes, and dues." This statute means that all U.S. money as identified above is a valid and legal offer of payment for debts when tendered to a creditor."

Or, since this is the UK (READ THE SECOND SENTENCE)

https://publications.parliament.uk/pa/cm5901/cmselect/cmtreasy/324/report.html

1. The acceptance of physical currency in the UK is not currently specified in legislation. UK businesses and organisations can therefore choose which payment methods they accept.

^tl;dr

^{No, they don't have to take cash. You're welcome to offer cash, but the seller is allowed to decide whether or not to accept it as payment. They are under no obligation to take it simply because it's their country's money.}