Consider the following situation: a democracy cracks down on a wave of peaceful street protests against its elected president, citing the constitution and the rule of law. The protests are illegal. Unconstitutional. The protestors undemocratic. Legal methods are found for making protest even more difficult. Some of the street protesters comlain that the protests should be permitted. A government spokesman responds that if the protesters want to be allowed to protest, they should try to get the constitution reformed (a process made practically impossible by the fact that the ruling party has an absolute majority in both houses of parliament, and the constitutional and supreme courts both generally agree with the government). Commenters mutter that protest doesn't have anything to do with democracy. That in a constitutional democracy like theirs, universal suffrage and the rule of law are what counts. That maybe the army should be sent in.
Who you consider to be right in a situation like this might well depend on your understanding of the possibilities and limitations of constitutional democracy. It's true that the protests are against the law. It's true that avenues of action exist for the protesters, but also true that they are practically useless. It's true that a basic or universal right seems to be threatened by the constitution itself. But is the right to protest really inalienable ? Isn't it accepted that the right to protest is curtailed in most democracies one way or another? Couldn't you argue that protest is inherently undemocratic? What about the people who feel scared when they see a protest march?
How should the government act, then? Should it maintain its position: 'rule of law trumps all'? Should it toughen its stance and jail the ringleaders? Or should it look for a negotiated settlement? The choice is between two forms of constitutional democracy: coercive and consensual. And it's a problem which most countries struggle with at one time or another, in one way or another. The decision the government goes for will generally reflect its ideological position: does it tend to liberalism and consensual democracy, and so want to negotiate? Or does it tend towards authoritarianism and coercion? But it will also reflect a calculation: is the section of the electorate which needs to be coerced big enough to cause problems for the government?
The right to self determination isn't the same as the right to protest. No rights are exactly the same. But it has interesting similarities in that few countries accept either right unconditionally. I don't think that any of us doubted that the PP would tend towards an authoritarian, coercive method of government when it was elected. We've seen multiple examples of this approach over the last few years (though to be fair to them, their abortion law reform was dropped – proof that the PP can be pragmatic when it comes to moral and ethical political issues, if not others).
I've written this to make it as clear as possible that when SCC/PP/whoever trots out the argument about the rule of law and democracy, they're really using a smokescreen. Every government has it within its power to push for a pragmatic solution to a problem like Catalan separatism if it chooses to. The PP has made a calculation that in electoral terms, ignoring Catalonia is the best policy. This is a political issue, not a legal one, and arguments to the contrary are misleading.