@Naxxy, I solved this and managed to pay monthly for Xbox Game Pass from Thailand. I just set my secondary MS account to have a UK address and my Thai debit card. I set my Xbox One S region to UK and then proceeded to purchase Xbox Game Pass and it worked.
So, for anyone else, not in a region supported by MS, try creating a separate UK MS account, add a UK address to it and add you local debit card (might work with PayPal too, not sure), set your Xbox One to UK region and then try to purchase again, using the MS account set to the UK, directly from the Xbox One.