What makes you think that the UK and EU are going to cease trading with each other ?
They won't. But it will be reduced. Clearly.
Even if you can get a free trade agreement - unlikely in the immediate short term - the extra bureaucracy will make UK companies uncompetitive in Europe.
But no one needs to worry as we will be again the Great Britain and China, Russia and the USA will be falling over each other to buy our goods........ Our pavements will soon be covered in gold.
Our ROW trade balance outside of the EU is positive where we export more in goods and services compared to what we import. The problem is the EU where we have a massive and growing trade imbalance to the tune of £100bn a year. If you look at how the main EU countries have always stacked the deck against us you would understand why. France has always blocked a single market in services which would have done much to restore our trade balance.
BTW in the 1980s & 1990s I exported to all three of those countries, especially the USA, along with about 50 others, including all of Europe including Eastern Europe, Canada, a few South America countries (where Spanish imports at lower prices into Spain caused a few, but solvable problems), Israel and a couple of other Middle Eastern countries. some North African mediterranean rim countries, South Africa, Australia, New Zealand, Japan, India & Pakistan. China was particularly difficult where things not only had to meet their technical standards but also pass their political censorship. About 85-90% of our turnover came from exports. Exporting is not difficult as trading is mutually beneficial to both parties, but getting paid was always much more challenging especially in some of the more far flung countries, compared to the UK.
One of the UK's strengths for the last 4-500 years has always been as a global free trading island nation and all UK governments still put great emphasis on exports with practical help, market assessments and their export guarantee scheme.