What we have to do is pretend that we hold all the cards, pretend that EU needs us more than we need them, portray any resistance to our demands as "punishing us" and blacken the name of any company who does the sensible thing and relocates to France or Germany. Then after the "negotiations" have "broken down", flounce out of the EU with a "hard Brexit", pretend that this is what we wanted all along, and pretend that the reason why the economy is suffering is that we are being "punished".
Then we can try to re-establish trade relations with Germany and act all surprised when they tell us that EU rules say they can't do that, we have to negotiate with the EU. And establish trade relations with New Zealand, and act all surprised when they remind us about the way we kicked them in the teeth 50 years ago, establish trade relations with the US and discover the precise meaning of "America First".
And our politicians will blame whichever EU official is villain-of-the-month for abiding by the rules of the EU, and the Brexit voters will blame the Remoaners for sabotaging the negotiations and the Remoaners will blame the Brexiteers for being such stupid buggers last June.
Meanwhile, our manufacturing industry will continue to decline, our farming industry will whine for subsidies to replace the EU ones, and our services industry?
There's two kinds of services. One kind is like haircuts. You can't import or export them, they have to be supplied at the point of demand. The other kind is like financial services. They dont have any capital equipment (apart from a bunch of cheap computers) but what they do need, is access to international financial markets, and regulatory permission to do business. Those, of course, will migrate to Germany, because without EU agreements, they won't be able to do business in the EU. At all.
Immigration will *not* get down to under 100,000/year, because it didn't pre-Brexit when Mrs May was Home Office, and we had total control over non-EU immigration, which was 257,000/year. So why would she be able to reduce that after Brexit? Maybe she could stop another of our service industries, the provision of education to foreigners who pay good money to come to university here. That would cut UK GDP, but it would reduce the immigration figures, temporarily.
Still, a country that is happy to cut off its own nose, would surely not balk at cutting off a few toes.
tl:dr - I'm not an optimist on this.
I know. Let's leave the British Commonwealth. That'll show them.