he should offer to assist in preparing for the negotiations to try for the best possible deal

But the point is there is no agreement on what the best possible deal looks like. Trade and immigration are a part certainly. But there is much else, which is where the hard, soft, red-white-and-blue, open, closed, business first, jobs first, and - the DUPs preference - 'sensible' come from. As I have said before, what is best for small independent businesses is not necessarily what is best for large national businesses and that can differ again from what is best for international businesses. Sometimes these are all aligned and we can sensibly talk about what is best for business in that situation. But no-one has the faintest idea how to address the cases where they are not aligned.

Then 'what is best for Britain' and what is best for business are not synonyms. Take air quality: Britain is frequently brought to court for failing to meet the EU standards. So we leave and are no longer bound by those standards. Is that 'best for Britain'?