The UK has a double taxation agreement with Spain so that you do not pay tax on the same income in both countries. Ask the relevant tax authority your questions about double taxation relief.
You should get professional advice on paying tax in Spain. Some English-speaking lawyer in Spain or registered ‘gestores’ provide this service.
Read guidance about:
tax if you leave the UK to live abroad
tax on your UK income if you live abroad
tax if you get a pension and live abroad
paying tax in Spain from the Spanish Tax Authority
EU factsheet on buying goods online from UK websites
Declaring your assets
As a Spanish resident, you must declare your global income to the Spanish authorities, no matter which country it came from. If you are not a resident, you will only pay tax on income that came from Spain.
You may need to file an annual declaration of overseas assets called a Modelo 720. There are penalties if you do not file, or give incorrect or incomplete information.