Michel Barnier made it clear for those who had not grasped it.
His mandate is/will be to negotiate the exit logistics of the UK from the EU, not a trade/econ EU-UK deal. That is a separate matter. And could take years.
Upon exit, no later than two years after Article 50 is triggered, thus in the spring of 2019, the EU-UK trade system that will apply will be the WTO one. Chances of a trade deal by then are, frankly, nil.
Which begs the question: The UK civil service, parties' think tanks and other think tanks had not thought all this forward? Paints a bad picture.