Rapp
There is no possibility of being able to choose where you pay tax. The law dictates where you must pay.
If one is tax resident in Spain then, just like USA, one must declare their worldwide assets in Spain
Exception are:-
A UK government pension is taxable only in UK (that’s a pension paid to a former government employee, not an O.A.P). Such a pension is ignored in Spain. And tax on rentals income generated in UK., that too is paid in UK
A UK government 'old age pension' is taxable only in Spain.
Double taxation agreements between countries vary from country to county but in general they are to ensure that you do not pay tax in two jurisdictions. So, for example, having paid tax on an OAP in Spain you do not have to pay again in UK. They have nothing to do with choosing where you pay.
There is no possibility of being able to choose where you pay tax. The law dictates where you must pay.