Theresa May has described the request for a referendum on Scotland before a final decision about Brexit as a ‘game’. There’s rather more to it than that. If Scotland votes for independence before the exit agreement is concluded it will materially affect the terms on which Scotland could become a member of the European Union. It would make it possible for Scotland’s status to be negotiated as part of the exit agreement; the precedent is the division of Denmark from Greenland, where part of a country left and part continued within the EU. That would also mean, under the terms of Article 50, that Scotland’s status was subject to majority voting rather than unanimous agreement within the Council. (We’ve been hearing a lot about the need for the EU to get the consent of all member states to agreement on the UK’s departure; that’s not actually required by the Article 50 process.) Delaying the timing of the referendum would have the effect of closing down both of those options, and while the situation could be resolved in other ways, a delay now could obstruct Scotland’s consideration for membership for several years.
The date is however a matter of politics, and if May wanted to scuttle Scottish independence, she has another option: which is, to offer an immediate referendum within the next two months, rather than one in 18 months to two years. The precedent is the short period permitted for the Brexit referendum; the Government’s rationale would be that this would clear the ground before the exit negotiations, Brexit in 2019 and the 2020 General Election; but the political calculation would be that a short time span would make it very difficult for the Nationalists to build enough support to win. The longer the delay, the more likely it becomes that the vote will be for independence.