It is you that misunderstands the term 'legal tender'. You can refuse legal tender for purchases, as I said above legal tender only refers to debts.
From the bank of England website:
Does legal tender status matter for everyday transactions?
No. Whether you pay with banknotes, coins, debit cards or anything else as payment is a decision between you and the other person involved in the transaction.
In addition, shops are not obliged to accept legal tender. If you hand over a £50 note to pay for a banana in your local grocery store, the staff are within their rights to choose not to accept it. Likewise for all other banknotes – it’s a matter of discretion.
http://edu.bankofengland.co.uk/knowledgebank/what-is-legal-tender/