The EU introduced a cap on such fees in 2015 after concerns they pushed prices up for consumers and unfairly burdened companies with hidden costs. While this price increase is 'opportunism' on MasterCard's part, I've seen people argue this isn't a consequence of Brexit, even though ultimately it is. British consumers are no longer protected by the rather "feisty" pro-consumer EU courts and are now at the mercy of the…
For example, in the UK Stripe charges one fee for European cards and another fee for non-European cards. I suppose this reflects the underlying costs. Because of this move by Mastercard I can see Stripe and others change this soonish with the lower fee applying only to UK cards (for customers based in the UK). And then we'll also have to see if the government keeps the cap for domestic transactions or decide to "cut red tape" and remove all restrictions.