Nicola Sturgeon has proposed to give Scotland, Wales and Northern Ireland a veto over the UK's exit from the EU. According to the BBC, she said:
If you look at states like Australia and Canada there are some circumstances where changes to their constitution requires not just a majority across the country but in each of the provinces as well.
The UK is not a unitary state it is a family of nations, it is made up of the four home nations.
We were told during the referendum that each of these nations had equal status, that our voices mattered.
If that is the case I think it is right that something that would have such significant consequences for jobs, for the economy, for our standing in the world, it should require the consent of not just the UK as a whole but that family of nations.
It's quite an attractive proposition. The Unionists clearly talked up federalism during the referendum campaign, and this is an obvious consequence.
If the Unionist parties don't agree, it's just another sign that they didn't themselves believe the very arguments they used to win the referendum. If the UK as a whole votes to leave the EU but Scotland votes to stay, it will therefore be prudent to hold a second independence referendum to allow the people of Scotland to chose which of the two unions they want to remain in, the British one or the European one.
While we're on this topic, the BBC's Laura Kuenssberg tweeted something rather party-political today:
Whether good or bad idea, anyone else think it's ironic that SNP want scot parly vote on leaving EU but only Scotland voted on #indyref?
— Laura Kuenssberg (@bbclaurak) October 29, 2014
A political union is best described as a club. It's always the case that members have the right to leave if they so desire -- it can never require the consent of all members. This was why the Scottish independence referendum took place only in Scotland and not in the entire UK, and this is why the Brexit will be decided in the UK and not by a referendum in the entire EU.
However, clubs and political unions can decide on their own rules for what they do collectively. They can decide that new members can't vote, they can decide who gets to join, and they can require unanimity for accepting new members or for joining other associations.
In most political unions the membership rules are described in the constitution or at least a political treaty, but because of the UK's unwritten constitution many of those rules are based on precedent and political statements. It therefore makes good sense for Nicola Sturgeon to point out that the Unionists' frequent talk about the family of nations has constitutional implications.
If Laura Kuenssberg wants to compare this proposal to the independence referendum, it would have been the equivalent of Shetland asking for a veto. However, nobody has ever described Scotland as a family of nations comprising Shetland and mainland Scotland, which is why nobody seriously considered doing this.