I think you should contact Microsoft and tell them what you want to do and let them step you through it. They will be happy to help you become legal - not just to make a legitimate sale, but because illegal software is often compromised with malicious code - that is, infected with malware. And infected Windows is not good for Microsoft or consumers.
To answer your question directly, if your UK credit card information is accepted by the computer, then not sure it matters what you are purchasing as long as it is a legal license. Surely there are Thai living in the UK that wish to use the Thai versions. I suspect there are French living in Thailand too, for example.
If you are wondering if the automated ordering process will cause problems if ordering with a UK credit card from a Thailand IP address, that I don't know. I don't see why it should. I would think if ordering on-line, you will prompted to select the version and language you want before you commit to the purchase. If you don't see a language option, then cancel before committing, and contact Microsoft directly and talk to a human being.
Also note that Windows 7 Ultimate supports 35 languages - but does cost more.
