(Tax)legislation changes all the time, what was correct last year may be different this year. I used to do it myself but now need a tax advisor. The best advise I can give you is use a good tax advisor (gestor). Follow the law and stay out of trouble.
Rent first and get to know how things work in Spain. Prepare well before buying.
My main home and family is in Spain. However, I don't spend 185 days in Spain but being a pilot I don't spend 185 days in any country. The 185 days is just one of many variables used to judge where you should pay tax.
Use a good tax advisor in the UK too. Also the tax year in Spain is different from the tax year in the UK which complicates things more. Sofar, I get more questions/work from the Spanish tax authority than from the British.