What will happen with UK suppliers that are now a part of the EU and will be outside of the EU after article 50 is enacted and the UK leaves the EU? Will they now have to register?
This has yet to be decided. Negotiations touching upon this are due to go on until February 2019, one month before this all comes into effect. If they decide to maintain a customs union, there should not be any disruption and goods should be able to move freely as they do today. If they do not maintain a customs union, then it will be a case of the UK exporting to the EU, meaning additional requirements. This process will be very protracted as nearly 50 years of regulations will need to be rolled back.