@Nina251
I'm no accountant, and certainly not knowledgeable about UK tax law, but the rule of thumb almost anywhere is that you pay tax on your profits.
Surely you already know that income from your properties in Spain is taxed in Spain, and there's lots of deductions.
If you end up with money in pocket ("profit") from that (after Spanish taxes), and that money goes to the UK, I'd presume you're required to declare it as income and pay UK tax on it, and there's likely some UK deductions you can take from that as well.
I expect any accountant in the UK could sort that rather easily.
Good luck with that. It should be a rather simple thing.