Many fintechs are registered in the UK as electronic money institutions (EMIs) 

Top ten UK-registered EMIs by asset size

Total assets for Airbnb Payments UK Ltd have been converted to GBP from USD using the official exchange rate for 5 January, 2021. Those of STRIPE PAYMENTS UK LTD, Skrill Ltd and Euronet 360 Finance Ltd have been converted from EUR using the official ECB exchange rate for 5 January, 2021.
Source: TheBanks.eu