No retailer has to accept anything. I can set up a shop in London that only accepts Scottish banknotes and not English ones. Or that only accepts Mexican Pesos or Bitcoin for that matter.

From the Royal Mint's website:

> Legal tender has a very narrow and technical meaning in the settlement of debts. [...] It does not mean that any ordinary transaction has to take place in legal tender or only within the amount denominated by the legislation. Both parties are free to agree to accept any form of payment whether legal tender or otherwise according to their wishes.