Worldpay was the biggest by far, but it never really shook off its legacy image (it had its origins in a couple of 1980s-era payment gateways). Stripe had overtaken it by the mid 2010s, and it has since been dismembered and partially rebuilt by (American) private equity.

The UK tends to punch well above its weight in FinTech, but PSPs are high-volume and very low margin so not really an attractive proposition unless you can be reasonably sure of grabbing a decent share of the global market.