A couple of months ago I posted here to ask about what to put on the form where it asks for the Thai tax ID number of the payer of the income in the case that the payer is the UK government. Not knowing the answer, I decided to wait until March, in the hope that the situation would become clearer. I finally filed this month using the online system with the help of a Thai friend. I am describing my experience in the hope it may help others in a similar situation.
I have been filing a Thai tax return since 2012, but this is the first time I have had to include foreign source income brought to Thailand. My assessable income consists of interest on deposits in Thai banks and UK old age pension. It totals a hair under 500KB.
It turned out to be simpler than I expected. I entered the UK pension amount in part 1 of page 2 of the PND90 online website form. It then asks for tax withheld (presumably they are interested in Thai tax withholding, not others?). I entered zero because no tax was withheld either here or in the UK. The website then offers the option of it being foreign source income, which I chose. The website automatically entered the 100KB deduction allowed (but not more than 50% of the income) and subtracted this. It did not require a Thai TIN for the payer of the income.
The website also automatically entered my old age (over 65) deduction of 190KB against my interest income. The various deductions (100KB + 60KB + 190KB) reduced my income to a hair under 150KB. Consequently, I do not owe any tax and should get back all the tax which was deducted from interest. I will post further here if this is actually successful, However, I have always received such refunds previously.
I didn't declare non-assessable income, which in my case is US social security, since this is clearly excluded by the tax treaty. I did not attempt to claim foreign tax as a credit towards Thai tax under a double taxation treaty---I do not know whether or not this is possible with the current online system.
I hope this experience is helpful for others in a similar situation.