Spain is a lot like the United Kingdom . . . a union of various historical nations. The only difference seems to be that the language of one is called “Spanish” rather than “Castilian” and the language of the other is called “English” rather than “British.”
‘British’ would be a Brythonic Celtic language similar to Welsh or Cornish. Though most of its dialects no longer exist, other than Northern Welsh and Southern Welsh. The Cornish language is the only other known dialect of the British language, though it is only known amongst academics and is functionally dead.
If George Washington had not defeated Lord Cornwallis in Yorktown we would still be speaking English in this country.