The UK pension is added to any interest from savings, dividends etc to calculate their worldwide income. Also any rental income or any income at all except from certain crown pensions.
If under 65, they will get a tax free allowance of €5,550 a year, at 65 this goes up to €6,700 and at age 75 to €8,100. Everything above that is taxable at 19% for up to €12.450 (over the tax free allowance), up to €20,200 then 24%, up to €35200 then 30%.
You can work it out on this tax calculator
caveat. I think that calculator is using previous allowances and hasn't been updated with this year's tax free allowances but will give you an indication.
As an example, I get a crown pension which is not taxed in Spain but is used as a guide for the level of tax and uses up your personal allowance. I am taxed fully on my state pension and private pension amounting to just over 17000 euro and was taxed €3,000 (give or take a few cents) this year.
This message was last edited by mariedav on 7/28/2023.