On 1 October 2017, Catalans went to the polls to vote on independence. Catalan independentists called for international, particularly European, support.
Brexit aimed to restore sovereignty to the United Kingdom. That rested on an interpretation of the UK as an internally integrated and externally bordered sovereign state.
The issue of sovereignty has never been resolved in Scotland. The 1998 Scotland Act, creating the Scottish Parliament affirmed that the Westminster Parliament is sovereign, but this is disputed.