There would be nothing in the British "constitution" that would make it unlawful to abrogate any treaties. Given an obedient Parliament there are no limits on what a British government can do. So long as the government stays within the law, as appropriately amended if need be, there is nothing the courts could do to stop it.

Where Mr Boris Johnson tripped up was that he did not stay within the law, and hadn't been able to change the law so that it allowed him to do what he wanted to do.

The only thing that in theory could get in the way would be a refusal by the Head of State.