Some thoughts prompted by the departure of Sir Ivan Rogers.
Here is a quick description of the circumstances that would lead to the best outcome from the Brexit negotiations: on the UK side, a political and diplomatic elite who have intellectually and emotionally bought into the case for Brexit and who have been preparing for it for years, all the brightest and best having spent years hoping and wishing for Brexit to happen and making detailed, together with a broader cultural and media elite who fervently wish for the negotiations to go well and who are prepared to lend a hand by producing the right kind of mood music and cheerleading to encourage the waverers and strengthen the resolve of the negotiators, and a populace beneath that wholly united behind Brexit, and cheerfully prepared to pay any price to make it happen; that combined with a European counterparty motivated only by magnanimity, one which has decided that the EU and the UK should be the best of friends, that the negotiations should create no bad blood but, on the contrary, that the terms of divorce should be so generous that the British might come to think that the EU is not so bad after all and truly is a beacon of sweetness and light in a dark world otherwise consumed with pettiness and rancour; and all of that combined with a profound and unspoken belief on both sides that restrictions on trade, whether on imports or on exports, are always and everywhere restrictions on the wealth and happiness of mankind.
Such a concatenation of circumstances is not wholly unrealistic. Something not a million miles from that must have lain behind the processes by which various dominions became independent from the UK. But it is clearly not the circumstances we are in.
If you start with that description of the first-best scenario then you can create a variety of second-best scenarios in which one or other element (a united populace, a generous EU) is missing. We are in none of those second-best scenarios either. We are some kinds of third best scenario in which we have some elements of the political and media elite, and a fair chunk of the populace, happy with Brexit. Given that background, if the terms of divorce are half-way acceptable then that would be a great achievement for our negotiators. The least we can do is to wish them the best of British.