How bad do you think this is going to be? The majority of trade for the UK is with commonwealth countries and the U.S. both of which are unrelated to the EU.
My bad, it was much closer than I thought. I do recall that the majority prior to joining the EU was with commonwealth nations.
Regardless, it's not like materials to trade disappears... my guess would be the UK negotiates with the EU to basically keep trade the same. If not the UK will be increase exports elsewhere.
It will be as bad as France, Germany, Poland and everyone else wants it to be. Decisions as to how to tax/regulate and/or block trade with Britain are now out of Britain's hands.
Those commonwealth countries (Canada) also trade with the EU. Whether they will choose to favour britain or not is also open. We'll get back to you on that one.
Pretty sure trade agreements with the commonwealth remain unchanged. Basically, you don't choose to trade, if an agreement is in place the companies choose to do trade. With a weaker pound it would be cheaper to buy goods from the UK.
Non-EU Exports for April 2016 were £13.0 billion. EU Exports for April 2016 were £12.0 billion.
Non-EU Imports for April 2016 were £21.9 billion. EU Imports for April 2016 were £19.1 billion.