He is making false assumptions. Again.
His fire service pension - in common with MOST UK Government Service pensions - is not exempt and thus not assessable if brought into the country (there are a few exceptions and a definitive list on the gov.uk website).
The UK State Pension (OAP if you wish) is not exempted by the DTA and is thus assessable. Tax paid on that, as with any other income, to the UK Government can be offset against any Thai tax liability. Unfortunetely, the UK Personal Allowance has no effect and ALL UK income is, if not exempt, assessable when brought into Thailand. I have not looked into it, but fear that any ISA income (from sales, dividends or interest) would also be fully assessable and the tax free UK wrapper would be irrelevent.
I am a couple of years off my OAP, but am hoping my military pension be "first" of my income so my Personal Allowance will be set against that and all my OAP will be taxed so I can set ALL of that against any Thai tax liability I may have. I ahve kept meticulous records of my assets as at 31 Dec last year and will be able to show my breakdown of which money I have brought in and I should not have any problems for several years - perhaps permanently - but planning ahead to try and mitigate anything as much as possible is, IMV, prudent.
PH