the fact that HMRC has deducted tax at source is as I keep repeating irrelevant to whether Thailand taxes that income. The UK DTA may exempt that income altogether as is the case with government pension schemes or will allow a tax credit against other forms of income. Using Thai allowances and deductions and full utilisation of available tax credits a basic taxpayer would only start paying Thai tax after remitting more than £39k per year. Sums below that figure would have a net zero Thai tax bill as the UK tax credit would cover it. You would need to get a TIN and file a Thai tax return in order to claim the tax credit.
Being paid out tax free in the UK is irrelevant to whether it will be taxed by the Thai Revenue Department. What matters is are you tax resident at the time you remit the lump sum into Thailand? If not resident then it’s tax free too in Thailand. If you are tax resident then the tax free lump sum from your private or company pension is 100% assessable for Thai tax. You cannot get relief from the UK DTA as you have no tax credit available to you. If the pension lump sum comes from a government pension scheme then it can be remitted tax free into Thailand.