Without an agreement Joelsnan, on future trading relations — particularly some kind of free trade agreement — trade between the UK and the EU will be based purely on WTO terms. There is a difference between terms and rules.
The duties and controls will this impose on trade between the UK and the EU will impacts mainly on agriculture and industries that depend on products which repeatedly cross between the UK and the rest of the EU, such as components to make cars (who we are already frightening away) or ingredients for processing food.
On top of that, the UK would lose the benefit of free trade agreements it now has with countries such as South Korea and Canada as a member of the EU. We would have to start again and negotiate with them. Who do you think they will give the best deals to? A large body like the EU or little old GB? That's why we joined the EU in the first place.
With more British imports and exports would facing tariffs it would means UK services, which can currently access the whole of the EU’s single market (i.e. currently, the 28 member states plus Iceland, Liechtenstein and Norway) relatively freely, would only be allowed the much more restricted access of the EU and UK’s commitments in the WTO. Trading only on WTO terms is the default position, but in fact, no country does it. This is why, as Whitewaves article explains, "It is not an end-state but the start of a process."
And how do you think those negotiations will go? particularly if we have refused to pay our bills as some suggest we should. Do you think we would be welcomed with open arms and deals be offered that are advantageous to us? That's another pig I see flying over.