If you are resident overseas, the taxation of your pension income will largely depend on what type of pension is being received and the tax rules in your country of residence.
If you are resident in Spain, your UK State Retirement and Occupational Pension income will be taxable only in Spain, and not in the UK, under the terms of the UK/Spain Double Tax Treaty. There is a small allowance available before tax is payable.
UK Government Service Pensions will remain taxable in the UK and will not be taxed in Spain.
Income from your UK Personal Pension will be taxable only in Spain and not in the UK. Generally, it will be added to your Spanish income and taxed at the appropriate rate, however, with careful planning the amount of your pension income that is liable to Spanish tax can be reduced significantly.
If you receive your pension lump sum whilst resident in Spain, however, it is taxable in Spain. The taxable amount is calculated as the difference between the capital received and the contributions you made and this ‘income’ will be taxed at 19% or 21%, depending on the total amount of ‘savings income’ you receive that year.