Two reasons can explain the absence of a payment method at an online casino. The first one is that the payment method doesn’t fulfil the Commission’s requirements and thus can’t operate in the country. The second one is that the casino you chose simply doesn’t offer the method because they don’t want to or because they can’t guarantee its application on their platform according to the UKGC regulations. However, if your method is well-established and is available on other sites in the UK, then it’s probably the latter.