You will be paying tax in Spain as of the moment you take up your residency. You can, however, be living in Spain and still pay in the Uk. This is the case where you spend less than 183 days in Spain. However, there are many thousands of EU citizens living in Spain who have never applied for residency. This is an irregularity, but I know people who have resided for 20 years and they still pay their taxes in the UK.
In your case, once you become a resident in Spain, you may still have to pay the income you derive from your real estate activity to the UK Inland Revenue, according to the Double Taxation agreement signed between Spain and the UK of 21 October 1.975.
This means that you would pay in the UK and not in Spain, as per the agreement.
Let me know if you need more help.
