there's now way VISA isn't the most widely used card in the UK surelyThat map has to be a mistake surely! The amount of places in the UK that refuse to take Amex makes it insane to think its the most popular card here..
VISA dominates in debit cards according to a quick googleThis seems to say in 2018,58% of credit cards were Mastercard in the UK and 36% were VISA:
https://www.ukfinance.org.uk/system/files/UK-Card-Payments-2018-Summary.pdf