Yes, England retained the nationalities of the kingdoms it absorbed and became a multinational state, whereas Spain followed more absolutist traditions and wanted not just fealty but complete assimilation. Scotland remained a nation after regal unification, whereas the nationhood of Catalonia is past tense, and therefore secession has significantly different legal ramifications in their respective empires.

"Catalonia had been a nation since the 10th century."

Again, if you believe that fantasy is true, is impossible to open your eyes to reality.

Please, point to a reputable source.