NHS pensions are not all crown pensions. This is an extract from “PAYE81025 – PAYE operation: double taxation claims submitted by non resident individuals: government pensions”
“Please be aware that for double taxation purposes, the National Health Service (NHS) pension is not classed as a government pension if it is paid by CAPITA or the Paymaster Generals office. If the NHS pension is paid by a local authority, it is classed as a government pension.”
So on the above it will depend whether your NHS pension will be taxed in Spain or not. In both cases you will need to declare it on your RENTA income tax declaration, but if it´s classed as a government pension and thus tax has already been paid ´at source´, so in the UK, you will need to declare it in a different ´box´ from other (pension) income, so that it will be taken into account to calculate the taxpercentage due on any other income, but it won´t be taxed double.
For RENTA 2016 (declared in 2017) it was box 480, but this can vary from year to year.