The market of 450M people, a $19.7 trillion GDP, and 27 countries... needs the market of 65M people, a $2.6 trillion GDP and one country, more than the other way around.
Does that makes ANY sense to you?
(Always remember: all the brexiteers promised us a "better deal" with the EU)
Especially when basic geography means UK trade will always rely on those countries most of all.