Put your finger on any festering territorial tiff just about anywhere in the world and there's a good chance that we Brits have had a hand in it. (Guatemala vs Belize and all that...)
The place of Catalans within centralising Castilian Spain has always been a bit of a wrangle, not least because they started out attached to Aragon, the side of the foundational pairing that didn't fork out for Colombus's speculative adventure west.
Yet if one were to ask the question of why there isn't an independent state of Catalunya today, without trotting out all the familiar tut tuts about 20th century ideological commotion, the fact is that there undoubtedly would be, had we Brits not shamelessly backstabbed our then allies in the War of the Spanish Succession during the lengthy horse-trading process which followed it — known to historians as the Peace of Utrecht (1713-15), but to the Catalans as The English Treason.
But hey, we got Gibraltar.