Can you really oppose secession but support the right to secede?
When Spain's constitutional court imposed limits on the extent of Catalonia's autonomy in 2010, and people in Catalonia started to get angry, the sensible response for the Spanish government was not to cover their ears and insist that nothing could change. When political parties in Catalonia tried to find a legal means to register their growing dissatisfaction, the correct thing to do was not declare it all illegal and hope that it'd go away. And when they held a referendum anyway, the Spanish government's moral options did not include large-scale violence against peaceful voters and demonstrators.
So far, so obvious. The natural progression when someone says sentences like these, as Owen Jones does in the Guardian, is to a declaration that even if we don't support independence, we should support the right of Catalans to decide. Jones calls national self-determination a "basic democratic principle", and he's far from alone. But he's wrong. National self-determination is just nationalism with a smiling face, and troubling in all the same ways. We should accept it as a principle only if independence is actually necessary.
If the people of Chelsea and Knightsbridge banded together and announced that they wanted to be independent from the UK, we wouldn't have to take them seriously. That still doesn't mean we should send in the Met to beat supporters of independence, but clearly this randomly constituted group of people do not have a right to their own government just because they want one.
The 'divorce' metaphor for secession is very misleading on this front. Everyone has a right to end a marriage without needing to spell out detailed reasons: it's just their decision, and they get to make it whether or not we think they should. That's partly because it would be traumatising and unfair to make people argue their case for ending a relationship in court in front of the other party, but fundamentally because it'd be wrong for the government to force anyone to live in an intimate relationship they don't want to be in. But is it wrong for a government to make somebody live under its jurisdiction, if they'd rather not? No: that's almost by definition what governments do. It's what conservative governments in the two countries I can vote in have been doing for the last four years, and although I am really not a fan of either that doesn't mean I have a right to band together with other left-wing people in North London or inner-city Melbourne and opt out.
You aren't subject to a government's laws because you agree to be or because you support it. The laws apply to you if the state is democratic and it doesn't oppress you. That's why Jones is right to say that independence really matters when it's a way of liberating people from oppressive rule. Decolonisation or independence for Kurdistan are important for exactly that reason: they're movements of people governed by an oppressive state which doesn't listen to them and whose legitimacy there's no prospect of salvaging. They don't and didn't get their force from a 'principle' that says a randomly constituted group of people has a basic right to decide who governs them.
People in Catalonia, of course, are not a randomly constituted group of people. Catalans' desire for independence is not the same as a hypothetical movement of Knightsbridge and Chelsea residents. But the way that it's different is one that shouldn't make it special. It's different because Catalans are an ethnonational group, with a distinct language and history in their region. When you spell it out, it starts to sound less attractive. Global politics has been rocked in recent years by the spread of nationalist ideas: that ethnic majorities have a right to preserve their way of life or demographic position, or that state borders should align with ethnic differences. These ideas are not new, and they are not good. But they are only a short skip from the view that national groups, because they are national groups, have an inalienable right to self-determination that other groups don't.
I don't know enough about Catalonia to know whether the region should be independent. If the actions of the central Spanish government have really been so consistently and irreversibly oppressive that independence is the only way out, then so be it. In the meantime it's important not to dignify as a basic democratic principle the idea that shared ethnicity, language and cultural history give groups a special set of political rights.