I'm aware of that. I wasn't giving legal taxation advice nor the classification of whether British citizen or not. I've always done my own self assessment but now use an accountant. The full details are more complex. My accountant handles UK and overseas client accounts so anything to do with living abroad I would get further advice from him.
Thai embassy London "Multiple Entry Tourist Visa will be valid for 6 months from the date of issue. Travellers must enter Thailand within the validity of visa, and they will be permitted to stay in Thailand not exceeding 60 days for each entry within the period of 6 months from the date of arrival in Thailand."
"because of the risks of physical dependence, opiate addiction and overdose, and it has become a controlled drug in many countries." I use paracetamol and meloxicam for arthritis pain and joint issues.
How many months are you coming for? Bring what you need plus a bit extra, get a letter or copy of prescription is a good idea. Check what sort of pain relief it is, some may need specific permission.