Thats odd they won’t take cash?
yes it is illegal to charge a card fee.
Copy and pasted from the citizen advice web site.
“Previously, retailers were allowed to charge up to 20% on top of the price of goods whencustomers paid with debit or credit cards or smartphone payment systems. However, as of Saturday 13th January, this was made illegal. … The ban will apply to all card payments, even if you use Paypal or Apple/Android Pay.”