That’ll be the subject of debate tomorrow at the American Philosophical Society in Philadelphia, of all places. They’re flying in a group of British barristers to square off with prominent American lawyers, and then they’ll let the audience decide.
Want my personal answer? Of course it wasn’t legal, not if we’re talking about the laws extant in the British colonies at the time. Independence happened because enough Americans became convinced that their rights weren’t secure within the framework of the empire, and that no remedies for their grievances were possible within that framework. That’s why the Americans made the case for independence from a standpoint of natural rights, rather than customary British rights. Sure, some of them originally argued against the policies that caused the whole uproar by referring to the rights of Englishmen, but the British government didn’t buy it, and neither did increasing numbers of colonists who eventually jettisoned those arguments and turned to natural law to justify separation instead.
Now, if you’re going to make a case that independence wasn’t kosher with regard to natural law, you’ll have to take it up with Jefferson, Paine, Adams, et al. And you’re going to have your hands full, because those guys can argue with the best of ’em.