If you have been asked for a Zipcode instead of a Postcode on the payment screen this is either because you are using a card that was registered in the USA or more likely because your phone is currently set to USA under 'Language & Region'.
Don't worry, it's very easy to change to the UK. On iOS devices, just need to go to:
Settings - General - Language & Region - Region
Specify the UK as your chosen region.
Please don't hesitate to talk to us on chat if you have any problems though 😁.