Even the Castillian speakers in the region spell it Catalunya after how it's spelt in Catalan.
All the Spanish speakers in the region would be 90 native born
catalanes and would mostly likely spell it
catalunya -- except for the fact that until 2006,
catalán wasn't recognized by the government as an official language and therefore couldn't be taught in schools, used on official signs etc. Good news, both variations of the spelling sound exactly the same.
Not trying to troll or hate... just find it fascinating that you learned it as
catalunya. It doesn't even show up as a word in official Spanish dictionaries. Props to you for knowing the
catalán spelling though.
EDIT: Be careful with the word Castilian as well, as it "technically" refers to anyone born in either the
castilla la mancha or
castilla y león provinces and the dialect of Spanish they speak there. And in this particular case, the
catalanes would probably not like you calling them
castellanos.
And back to all the Autonomy being sought by everyone. Sure they want more freedoms (like being able to teach in
catalán) but succession would never happen because they'd lose too much. It's only hardcore zealots that want complete independence.
And back to the OP, I'm sure UK isn't the only country that does this frequently. Spanish customs hassles anyone 1st, not speaking Spanish and 2nd even more so if you're not from a EU country or the US and 3rd even more so if you're not white. I hate to say it, but its true.