Law is not a functional programming language, in which we insert facts, turn a handle and get an answer. The law is essentially what you can successfully argue,

Law is not a functional programming language, in which we insert facts, turn a handle and get an answer. The law is essentially what you can successfully argue, bolstered by existing law and case history and all the rest of it. "in the absence of British law, default to EU legislation" would by no means be any kind of clarity, and I would anticipate such a thing would give years and even decades of discussion and argument in the various legal forums and courts to interpret that in all the various permutations and possibilities.