If you are living full time in Thailand, you become uk-non-resident the moment you left UK. It is none of the UK's business where you are. Generally speaking if you don't live there and aren't using their government services, you should not be paying tax there.
Apply for an NT tax code via https://www.gov.uk/government/publications/income-tax-leaving-the-uk-getting-your-tax-right-p85
From what I've seen you will only successfully get a Thai tax code if you are actually going to start paying tax in Thailand. There is no need to do this. Thailand is not trying to tax foreigners on money coming from outside Thailand (in general).
I'm sure you will want more proof than just the above, but I'm trying to point you in the right direction. Its a grey area I've spent many hours looking into and have discussed with 5 or 6 international tax advisors, albeit not specifically pension-related. I'd be interested in replies to the contrary.