If it's an occupational government pension (Civil Service, military, police, NHS, etc), it will be covered by the UK/Thailand DTA and, hence, not assessable for taxation purposes in LOS. However, I think it more likely, from the what the OP has said, to be the UK State Pension which is not explicitly referred to in the DTA and so, erring on the side of caution, should arguably be treated as assessable income. And then there is the issue of the income he is apparently earning from online work locally, which might, in itself, justify the need for a TIN in his case in any event.