Why is it therefore so hard to grasp what would happen to trade and tariffs if the UK leaves the EU (one set of clearly defined rules) and enters the "not EU" (another set of clearly defined rules)
It's not hard to grasp, but it's a reflection of the world as it is now, not the world as it would be after a negotiated Brexit. Have a bit of imagination Edwin
Or, a negotiated remain having had a Brexit vote raise EU-wide concerns about what a UK-less Europe might look like.