Seems you need to use multiple providers and route payments depending on card type if you want lowest rates. For example, mollie is 2.5% for Amex but Stripe is 1.5% for Amex but or others it swings other way.

> Mollie for UK

- 1.2% + 20p for standard cards

- 2.90% + £0.20 for corporate and European

- 2.50% + £0.20 for Amex

- 3.25% + £0.20 for international

> Stripe for UK

- 1.5% + £0.20 for standards cards inc Amex

- 1.9% + £0.20 for corporate uk cards

- 2.5% + £0.20 for international within Europe

- 3.25% + £0.20 for International + 2% if fx conversion

[0]https://www.mollie.com/gb/pricing#psp-block

[1]https://stripe.com/gb/pricing. (scroll down for list)