You would need to request this from the Hacienda and then send it on to HMRC as requested. The Fiscal Residency Certificate also exists without ´Convenio´ but that is not always accepted. On the form to apply there´s a separate box to tick for ´Convenio´ and drop-down menu for countries to choose from.
You need to have submitted at least one tax return in Spain. If you have then they will issue a “Certificate of Fiscal Residence”. If not, then you will have to submit a return and pay the tax before Hacienda will issue the certificate. Once you have done that, then they will issue a certificate, and you can reclaim the tax paid in the UK, and ask HMRC to authorise your pension to be paid NT (no tax). The refund takes about 6-8 weeks, but there will be no problem in receiving it. You should submit your claim for a refund and NT on a Form Spain Individual