If you have P60's covering funds coming from the UK into Thailand, I think you will be safe from Thai taxation.
I have 2 pensions coming into Thailand and have P60's and Statements of future payments, The P60's show annual amount of income and tax paid. The Statements of future payments show the monthly amount and the tax deducted.
State Pension, I have no idea as I cannot claim it yet.
Any monies remitted to Thailand that has not been taxed, or you cannot show where it has been taxed, will most likely incur Thai taxation.
O&G workers for example, working in tax free places and having their wages paid direct to Thailand.
I think many people are panicking and doing Chicken Little impressions over nothing.