In what sense do you see the Bill of Rights as unassailable and immutable, unless just a theoretical sense?
Now the divine law of justice itself, on which the principles behind the Bill of Rights are based, is unassailable and immutable. But this would be so regardless of how the boundaries around tax jurisdictions are drawn on the globe. The Bill of Rights themselves, on the other hand, are manmade laws that are no less susceptible to manipulations of politicians and judges than manmade English laws were. And if anything, I would posit that the very concept of common law we inherited from our British pedigree has a better philosophical underpinning in the unassailable and immutable divine law of justice than the constitutional amendments that were written and ratified by politicians we call the Bill of Rights do.
In fact I think British colonies often had a great deal of liberty and independence. Consider Hong Kong. I think the American colonies, especially given the culture of their population, had potential to be relatively free as well, had they remained colonies, perhaps more free than what they very rapidly degraded into after they had replaced that one tyrant 3,000 miles away for the 3,000 tyrants one mile away. The enlightenment ideas that are enshrined in the founding documents didn't depend on those documents for their strength or validity.