You're ignoring the most difficult issue in a trade agreement. It's not the tariffs or the lack of them which is the tough part to reslove but the regulatory standards of the goods to be imported & exported that needs to be decided.
For example the EU banned the import of chlorine washed chicken since 1997 which of course hurts the US, and who are going to be very keen to include it in any new agreement with the UK and will use it to barter prospective concessions in exchange . As a nation we're going to first have to decided whether that is acceptable or not and that alone will be difficult enough to resolve. And that's just one product.
This has all the makings of a category 5 $h1t storm that will last an eternity.
28 of the 32 wealthiest countries in the world reside within the EU. Realistically we will need the US because they are one of the few remaining countries outside of the EU that would be worth doing a deal with, but as discussed on here previously, the exact agreement we strike with the US is open to debate. Should we still want a trade deal with the EU we would need to align our standards with them, which makes a deal with the US harder.
Also 80% of our economy is services. Try selling financial services to a person in India... or try sending engineers half way round the world just because some numpty has decided trading with the countries geographically closest to us wont work anymore because of immigrants and sovereignty.