Hi Stephen,
The tax treaty between the UK and Spain:
2014 UK-Spain Double Taxation Convention
Allows for UK government teacher's pensions to be taxable only in the UK.
INTM343040 - DT claims and applications - Types of income: Pensions and Annuities
As a UK national, you are entitled to claim personal allowance no matter where in the world you live.
Where the personal allowance exceeds your pension income, then no tax is payble.
Income tax rated in the UK are 20%, 40% and 45%.
Thank you.