that’s correct you must first become Thai tax resident. Any income received prior to the tax year in which you become tax resident will not be taxed on remittance.
if you switch in the 2nd year to the 65k monthly requirement and reinvest your 800k back in your home country then your maths falls apart. After 5 years you would be losing money and still be on a fraudulently obtained extension of stay.
yep you are missing the fact that he is a Thai tax resident on a frozen state pension. To permanently unfreeze his frozen pension he must divest himself of current Thai tax residency and gain residency in another compatible jurisdiction. 183 days in the UK in a tax year will do that. The question is not about paying tax on the pension but about unfreezing a frozen state pension. Once he gets UK tax residency if he returns to live in Thailand his pension will again get frozen but at the uprated rate.
He must first become UK tax resident so stay 183 days in the UK in a tax year. Be careful with the dates. Once tax resident then if he returns to Thailand his pension will be once again frozen but at the current higher rate. Make sure you inform the Pension Service of his travel dates as he is entitled to the uprated pension from day 1 of entry into the UK.