If you stay more than 180 days in Thailand you automatically qualify as a tax resident. However, you do not need to register unless/ until you remit into the country an "assessable sum". Then you have 60 days to register and must submit a tax return before the following April. No "assessable sum" equals no need to register. That is the current law. What qualifies as an “assessable sum” is determined by many factors, not least the Double Tax Agreement between Thailand and your country.
UK State pension is taxable in Thailand.
This group may be useful
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All that said, the law as it stands is not actively enforced. Hardly enforceable. The tax people need to know who is in Thailand, how long they have been here and what monies they bring in. As neither the Immigration Dept. nor the Banks provide that information, they can only rely on you volunteering it. That may change, but there's little sign of it yet.