Nicola Sturgeon said tonight she was determined to hold a vote on Scotland's future on the reasonable timescale she has already set out, and that she would "consider her options" if Theresa May tries to block it (which is clearly what will happen). So what are the options for holding a vote without a Section 30 order, and which is Sturgeon most likely to plump for?
1) A consultative referendum without Westminster permission. This would probably be the best option IF any legal obstacles can be overcome (opinions differ on how easy or difficult that would be). It would essentially be lose/lose for Theresa May - if the Tories and other unionist parties actively campaign in the referendum, they will give it legitimacy and they may as well have just granted the Section 30 (and they will also look stupid and anti-democratic for having not done so). But if they boycott the referendum, a Yes vote will be assured, quite possibly on a respectable turnout, and the moral authority of the 2014 result will be surrendered.
2) An early Holyrood election to obtain an outright mandate for independence, with the SNP and Greens placing an explicit commitment to independence (without any need for a further referendum) in their manifestos. This option has the beauty of being legally watertight - there's nothing London can realistically do to stop it, short of dismantling devolution. It would probably mean we'd be chasing 50%+ of the popular vote on both the constituency and list ballots, which is a very tough target - but remember the SNP and Greens took an outright majority of votes in Scotland at the UK general election in 2015.
3) SNP MPs or constituency MSPs (or both) resign en masse, and trigger by-elections across Scotland to obtain a mandate for independence. This idea has been floated a few times, but is unlikely to happen because a Scotland-wide mandate would be required to bring about independence. The SNP (and allied "independents") hold all but three Scottish seats at Westminster - but those three would be enough to ruin the legitimacy of any mandate, unless the vote obtained is implausibly decisive. As the option of a Holyrood general election exists and is superior, there's simply no point in going down the by-election road.
4) Play the long game, implicitly accept Theresa May's decision, and wait until 2021 to obtain a mandate with which to beg her (or her successor) for a referendum all over again, with no guarantee that she will prove to be any more reasonable. This is exactly what May and Davidson want us to do - which might be a little clue as to why it's a very, very bad idea.
Verdict : Obviously it'll either have to be the consultative referendum or the snap Holyrood election. The only other way forward I can see would be an early Holyrood election to obtain an even more emphatic mandate for a Section 30 order than the one we already have - but if Theresa May is just going to keep mindlessly saying no, what's the point? We'll have to take the bull by the horns eventually, and dragging voters to the polls one more time than is strictly necessary might prove counter-productive.
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Random thought : Is Scotland the only "democratic" country in the world where the defeated opposition leader gets to announce what the elected government won't be allowed to do?