IANAL, this is what I've heard, open to criticism:

Legal tender in UK legally only affects repayment of debts: they are obliged to accept legal tender for repayment of a debt, within reason. So you can always pay by cash e.g. if you eat in a restaurant, legally, as you are indebted to them after eating your meal; although they don't need to make it easy, you'll have to have a day in court to enforce this in practice.

Nobody's obliged to offer goods for cash, so you have no obligations in a shop, unless they gave it to you on credit.