183 day rule

If you spend more than 183 days in Spain during one calendar year, you become liable for Spanish taxes whether or not you take out a formal residence permit (Residencia).
These days do not have to be consecutive. You do not become resident for tax purposes until the morning of the 184th day. Temporary absences from Spain are ignored for the purpose of the 183-day rule unless it can be proved that the individual is habitually resident in another country for more than 183 days in a calendar year.
This is not negotiable.
If you still pay taxes, think you live in the UK, have all your family there, etc. It does not matter. You still have to pay Spanish taxes and are considered a resident in Spain.
The good news is that you don’t have to pay UK taxes, if you do, then that is your fault. Paying taxes in the UK or any other country does not exclude you from paying Spanish taxes if you are resident for 183 days in a year.
Not a Spanish Tax year is the same as a calendar year. 1st January to the 31 December.