Probably unlikely they would have a card machine to pay. Might be a time to dig the chequebook out, that way you can prove you paid and also know that the money is going through the GP practice account.
I personally don’t see how the amounts GPs charge can be justified. My experience was that I was advised the fee depended on what was involved, only for this to change to £110 flat fee (cash) when the medical had been completed. After initially saying they could keep the form, a £50 fee was agreed for the 10-15 minutes work (if that). I did pay in cash on that occasion, but asked for a receipt.
However, if a fee is agreed and the work carried out, I would pay at the desk voluntarily in all cases.