If you are not resident in the UK and in receipt of a UK based pension, then it may not be taxable in the UK. You will need to check to see if a double taxation agreement exists between your country of residence and UK.
You can see guidance below about some pension types that remain taxable in the UK. Those advising they are government pensions, remain taxable in the UK.
INTM343040 - DT claims and applications - Types of income: Pensions and Annuities
Where the pension is not taxable in the UK, then you will need to submit a DT-Individual form. You can download and print the form, complete all relevant sections and send to your local tax office in your country of residence.
Living and working abroad and offshore forms
They will validate the form and return it to you. You should then send it to: