I understand a fair point, uk debit card extremy handy to scan on Heathrow or Gatwick Express, I have a Thai mastercard not certain, regardless of unknown rate and overseas charges it'll work on uk transit systems, I prefer leave my thai cards in Thailand.For those who used to queu to buy or reload oysters, most transport in London don't take cash.
Can you use a QR code at ticket barriers Heathrow or Gatwick, probably the way foward ?