I don't believe that there is any connection between residencia and liability for income tax. As I understand it, if you are in Spain for 183 days of the year you must file a tax return - whether you are officially resident or not. Of course it may be that applying for residencia will alert the Hacienda to your existence!
If you do declare your income, tax paid in the UK will be offset against your liability in Spain. One of the concepts of the UK's double taxation agreements with other countries is that 'earned' income is taxed in the country where it is earned. My view is that if you are working from home in Spain for a UK employer your salary is earned in the UK. However, I think that the official line is that you pay tax in the country where you are when you earn the income; i.e. Spain. I suspect that if you explained your situation to the Centre for Non Residents in Newcastle, they would tell you to declare yourself non resident in the UK.
What do you do about health care? Presumably you are not paying Spanish social security contributions. Even if you do declare yourself resident in Spain, it may not be that easy to get medical cover. The following link is to a post which suggests that you can only pay Spanish SS contributions from income earned in Spain:
My plan is to say nothing in the UK and carry on paying tax and NI. I'll file a tax return in Spain, but hopefully tax already deducted in the UK will mostly cover the tax due. Medical cover I'll get through my wife who will be working for a Spanish company. It all sounds very simple - I suspect that it won't be.........