Is a Spain visa from UK valid for Spain only? Or I can visit other countries?

A Spain visa from UK (Schengen visa) is generally valid for travel throughout the Schengen Area, not just Spain. Once issued, you can visit other Schengen countries as long as your visa is valid and you respect the 90/180-day stay limit. The main rule is that Spain should be your main destination (longest stay) or first point of entry if time spent in each country is equal.