Hi Robert Nicholson,
HMRC do not provide certificates confirming that you are not resident in the UK, only that you are resident.
To claim UK tax exemption on your pension, you should submit Double_Taxation_Spain_Indiviual.pdf
(publishing.service.gov.uk) to us once your pension has commenced.
You should send this to us with your certificate “Residencia Fiscal en Espana Convenio”.
Please note however that certain pensions remain taxable in the UK and cannot be given the NT (No Tax) exemption.
The attached link refers.
DT claims and applications - Types of income: Pensions and Annuities