In the topic I’ve linked below and on the Web Payments SDK Overview docs, it says that the postcode field should appear automatically after the user enters their UK card number, however while setting it up in a sandbox environment, that functionality doesnt work even if I manually set the locale to en-GB. Is there another step I need to do to get that working?
This is expected since the sandbox test values are US based cards. We unfortunately don’t have a test value that will show Postal Code. If you’d like to test this in sandbox you can enter a valid UK card and not hit the charge button. It will correctly show Postal Code. 
I have been testing it with real UK card numbers, both a debit and credit card (not done any charge testing yet). the little card icon in the form correctly detects the cards as visa and mastercard but the form only shows three fields for card number, expiry and CVV even once completely filled.
This is the correct behavior. The form infers whether or not postal code, zip, or even asking for AVS is necessary. It does this based on the card number.