As of October 2026, Apple's list of supported countries and regions has about 100 entries, including the UK, Ireland and much of Europe. That list tells you where a card can be added to Apple Pay. It does not tell you where you can play. That depends on the gambling law where you live and on the licence the casino holds, and each casino decides whether to offer Apple Pay at all.
Great Britain has one rule to know. Since 14 April 2020, gambling businesses there have not been allowed to accept payment by credit card, and the rule extends to credit card payments made through a money service business. An Apple Pay deposit is a payment with the card behind it, so choose a debit card in Wallet when you pay a casino licensed in Great Britain.
Local rules apply to casino apps as well. Apple requires real-money gaming apps on the App Store to hold the necessary licences in the places where they are used and to be restricted to those places.