By my reading of the UK - Thai DTA, your army pension is non-assessable in Thailand (can only be taxed in UK) but the UK state old age pension is assessable in Thailand. So yes, to my understanding you should file, but only for the state pension income, and once the various exemptions & deductions are claimed, good chance you won't owe much if anything in tax. There is a personal exemption of 60,000 baht (120k if married and filing jointly), and if you are over 65 there is another 190,000 deduction. I have also heard, but you should veriify, that 50% of pension income up to a maximum of 100k is also exempt. Then, after all these exemptions/deductions are applied, no tax is owed o nthe first 150k. Consequently with UK state pension as your only assessable income you'd likely owe no tax.
You can claim a tax credit for any taxes paid in UK (don't ask me how, from this thread it seems the promised adjustment ot tax forms to include this has nto occurred). The tax credit is only relevant if you would otherwise owe taxes.