If HMRC don't send you an annual return, you then only have to complete one if you have UK taxable income (presuming you are a tax resident of Thailand?). Unfortunately, they send me one annually, so I have to complete it, even though my UK taxable income is below the tax threshold. If I don't, even though I don't owe tax, they would impose fines and penalties. I mention this because some people might not be aware of it.
As for double taxation agreements, they work okay if the UK tax is equal to or higher than the Thai tax. However, if the Thai tax is 30% and the UK tax is 20%, you might find that Thailand is entitled to claim the 10% difference. Again, when this matter is clearly outlined in Thai tax law, this is something that expats need to get advice on.