Hi
A State Pension is a UK National Insurance benefit and is taxable, but usually paid without deduction of tax.
To ensue that it is not taxable, please download and complete the DT individual form at
Double Taxation: Treaty Relief (Form DT-Individual) and send the signed and dated form to the Singapore tax office for validation.
They will return the validated form to you, so that you can send it to HMRC at the address on the front page of the form.
Thank you