Ex-pats can’t just decide where to declare. If you are in Spain more than 183 days per year then you are legally bound to declare all worldwide income in Spain.
Not so Katy. Under the EU double taxation treaty you can decide where to declare your income if that income is obtained outside Spain. You are still required to submit a tax return but not billed for tax.
Not so, if you reside in Spain then you still have to make a tax declaration to the spanish authorities. Any tax paid in the UK will be credited under the double taxation agreement but depending on an individuals circumstances then you could find there is more to pay in Spain. I should know, paid it for years.