Theres a transition periode until dec 31st 2020.
US has already made it clear, it will bend over the UK in trade negotiations. America 1st! Not just a empty slogan with Trump.
Food and Medicine shortages could happen, but for a specific reason: Currently the UK has only very limited infrastructure for customs checks. But once the transition period is over, everything would need to be checked at the borders. Unless the UK severly ramps up the infrastructure and workforce there stante pede, then imports will have to wait longer at the borders until the checks are processed and will pile up and get delayed as a result.
As for UK's trade deals and how easy they thought to get them, it really seems like they still believe they are that giant british colonial Empire that they were 100 years ago. But they don't anymore, and are much, much, much weaker than they think they are. Hence why most countries don't have new trade deals with the UK yet
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454