99% of debit cards are Visa, and most people buying stuff on Amazon would be using a debit card, folks only really use credit cards for large purchases and things like travel, here.
Even when it comes to credit cards, I would say Mastercard has like a large chunk - like 80% in terms of institutions (but who knows in terms of actual customers), I would say.
Barclays and HSBC are the only majors that do Visa as far as I know. And Vanquis which is a junky one for people with bad credit.
The other majors like Lloyds, TSB, Bank Of Scotland, Halifax, Natwest, RBS, are all Mastercard. Same with most smaller banks like Virgin and CapitalOne, and store branded ones like Sainsburys/M&S/Tesco, too, as well as the remainder of the popular bad-credit cards like Ocean, and Aqua. Mastercard.