Hi Alistar696,
Please be advised that with the new payment system, it detects your country and applies the correct currency and payment methods for that location, which can not be changed.
To see the payment methods available to you, just have a look at the checkout after you have added the Gold Premium Club to your basket. If you are still stuck, send us a ticket here:
https://support.runescape.com/hc/en-gb/categories/200977401-Payments-and-membership
I really hope that this has helped!
**Seasons greetings**
Mod Loopey