Is the Declaration of Independence Illegal?

This is an interesting story that I had not run across until now.  A debate was held in Philadelphia last Tuesday and centered on the legality of the Declaration of Independence.  This debate pitted a team of American lawyers on one side (not a good start) and a group of British barristers on the other.  Here was the case the Americans presented: 

The Declaration is unquestionably "legal". Under basic principles of "Natural Law", government can only be by the consent of the people and there comes a point when allegiance is no longer required in face of tyranny.

The legality of the Declaration and its validity is proven by subsequent independence movements which have been enforced by world opinion as right and just, based on the fundamental principles of equality and self-determination now reflected in the UN Charter.

And here was the British case:

The Declaration of Independence was not only illegal, but actually treasonable. There is no legal principle then or now to allow a group of citizens to establish their own laws because they want to. What if Texas decided today it wanted to secede from the Union?

Lincoln made the case against secession and he was right. The Declaration of Independence itself, in the absence of any recognised legal basis, had to appeal to "natural law", an undefined concept, and to "self-evident truths", that is to say truths for which no evidence could be provided.

The grievances listed in the Declaration were too trivial to justify secession. The main one - no taxation without representation - was no more than a wish on the part of the colonists, to avoid paying for the expense of protecting them against the French during seven years of arduous war and conflict.

Unbeknownst to the American lawyers, the Declaration of Independence was of course illegal--that is, under British law.  That's why Americans appealed to their rights as human beings instead of their rights as Englishmen.  While it is true that Americans years before the Revolution did at one time appeal to their rights as Englishmen, this argument gave way to an appeal to the "Laws of Nature and of Nature's God."  Thomas Jefferson's Summary View of the Rights of British Americans, published in 1774, makes this distinction rather clear.  Though Jefferson at the time was still trying to make an argument of prudence that was based on an appeal to the traditional rights of Englishmen, he nevertheless grounded that tradition upon standards outside of the positive law.

The British barristers had it absolutely right that the signers of the Declaration were committing treason; but again, they were committing treason in the eyes of the Crown.  Why else would the Founders mutually pledge their lives, fortunes, and sacred honor in signing that document?

The barristers, slyly using Lincoln to attack the "secession" of the colonies in 1776, instead repeated the same arguments used by John C. Calhoun who conflated the right of revolution and a supposed constitutional right to secession.  Lincoln always maintained the distinction which Calhoun blurred and saw secession as nothing more than sugar-coated rebellion.  Lincoln knew that Southerners in 1861 could not openly make an argument appealing to the higher law like the Founders had done in the Declaration because that law also would have supplied the slaves all the more reason to revolt against their masters.  Furthermore, Alexander Stephens, Vice President of the Confederacy, and Senator Pettit of Illinois, among other Confederate sympathizers, made arguments attacking the principle that all men are created equal, a principle of the higher law. 

Although the American lawyers appealed to natural law in arguing their case (an oddity considering the status of natural law in law schools today), they didn't seem to understand the distinction between natural law and positive law.