L o l at you implying we couldn't be better off and make an extra 1 + bil with trading with the world not just eu.
And what would the UK trade? The UK has a very small industrial base - 10% and that would be competing with Germany/France/Italy. Would it be trading its' service sector - that make up something like 80% of economic activity - the Primarks, Poundlands, Tescos, Morrison etc. Including the main driving force behind the UK economy i.e financial services which the UK already trades with the world. The UK has very little to trade with to the rest of the world.
Graphene - a wonder material invented and researched in UMIST is now financed by Chinese and EU money and controlled by the Chinese. Who when the UK asks for a trade agreement the first thing the Chinese will say is, 'will you be putting trade barriers to cheap Chinese steel'. If the UK trade negotiators say yes, the Chinese will show them the door. The same with the US - 'accept TTIP or there's the door.
The UK could trade their agricultural products with the rest of the world mind.
UK exports to the EU will carry a tariff as will EU trade with the UK. Which will mean EU goods will cost more thereby raising inflation, considering the UK imports lots of goods from the EU.
The economic outlook for the UK is not too rosy to put it mildly and that was whether the UK decided to stay or leave the EU. At least inside the EU the economy had little disruption to trade outside that will not be the case.