Right to self determination and constitutions

Hi All,

One thing that the Catalonia crisis has brought into focus is the tension between respecting a constitution and respecting self determination of national  groups without a state.   

You see many people are saying that Spain's courts have said independence by Catalonia would be illegal as the Spanish constitution doesn't allow it. And yet hiding behind such a constitution didn't stop American colonists rebelling against the British or the Empire of   Austria - Hungary being dissolved or the ending of French rule in Algeria  (which was constitutionally speaking seen as a part of metropolitan France and not a French colony in north Africa).  

The arguments put forward by groups who want independence from an Empire or from another state would doubtless site the history of national self determination which gained popularity since Woodrow Wilson and Versailles and especially after world war two de colonisation of Africa and Asia.  It was and is after all a key part of Zionism that the Jewish people have the right to self determination and a separate homeland.

So who is right?  Those who say that  a nation's constitution is indivisible and  can never change or is the wider principle of national self determination trump everything ?  And what do we define as a potential nation? Would an independent Republic of  London or Yorkshire or South Dakota be less worthy of independence than Catalonia or Scotland?