I wonder, I mean the real point being made, I think, is this, that it is very simple. There is a common law rule that the Crown cannot, under the prerogative, alter the law of the land. EU law is the law of the land; therefore the prerogative cannot be used to alter the effect of EU law in the United Kingdom. That is the synergism.
As I understand it, your analogy with the Dangerous Dogs Act is designed to illustrate that EU law is not the law of the land in the same sense as the Dangerous Dogs Act is. You cannot under the prerogative alter the Dangerous Dogs Act; you can, you say, alter EU law precisely because it is not part of the law of the land in that sense.