Same reason I want the US off the podium: it might be crap at the start, but depending on a single benefactor is never a good sign. I want to be able to say "well, I don't like it! F*** you!" and look for someone else.
You realise that you cant just always go trade with other people right?
Others might not want the goods you sell, or not value them as much as your current partners do.
Also theres the logistics of it, some goods cant handle going half way around the world, if their processed in the UK, or might spoil on the trip.
This results in productiong moveing away from the UK (loss of jobs) and restructing of what it can trade/produce.
Overall its gonna have a impact, when you transition.
Thats before you have to factor in that in the EU theres alot of protections, that mean everyone that trades, do it on same terms.
In the US the UK will have to compete with lower wages, GMO crops, ect ect.
This will result in less gains pr item traded. (or lowered wages, so UK can compete)
The UK is gonna be alot poorer for its "right to choose, where it goes", because ironically the terms it had in the EU was pretty great.
Also the US had terms, when they proposed a trade deal.
From a position of strength, the US is gonna screw over the UK in any deal they make imo.
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071