It has been stated that the UK is a voluntary union. If this is democratically the case I think there should be a legal mechanism for any part of the union to decide whether or not to leave that union.
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In 2014 an independence referendum was held in the nation of Scotland. At that time it was decided via referendum that the majority did not wish for independence. Various things have happened since then, and some people would now be eligible to vote who wouldn't have been old enough before. I believe Scotland should be able to decide to do this again if it wishes to. Northern Ireland has statutory provisions for a referendum on its constitutional future. I think Scotland and Wales should have statutory provisions so that they do not have to seek permission from Westminster to hold independence referendums.