Once you are a citizen of the the country you want to pay tax in, follow the procedure in the HMRC Double Taxation Agreement for the country in question (all available on line).
This will involve providing proof of citizenship and the necessary certification from the tax agency in your new country on a specified form - in my case IRS in the USA.
Send to HMRC.
Once agreed, you will get a confirmation letter from the HMRC with advice that you need to inform them if you change your residency status.
HMRC will automatically process the refund for any part year already paid in the UK.
HMRC will send a "NT" (No Tax) code to Equiniti.
You will need to inform any UK banking institutions that deduct tax on interest.
Note that Equiniti are likely to screw up and start charging tax when any additional pension kicks in - in my case RFPS05 for FTRS after AFPS75 for Regular service I was already receiving at NT.
If this happens, they will try saying "we are only doing what HMRC told us". Call HMRC, explain what transpired - quote your NT confirmation letter - and they will e-mail "NT" notification to Equiniti again. Sorted within a few days.
All the above took about 4 months (6 years ago) - the main log jam being the queue for the IRS certification.
Last edited by RAFEngO74to09; 22nd Aug 2020 at 15:28.