Of course when we leave the EU it will no longer be the biggest market in the world, that will now become the USA, with whom we will immediately do a deal. We will still do a deal with the EU, but will be free to do many more deals quickly with other countries. We will also retain the trade deals already set up by the EU to which we are sovereign signatories. The overall picture is that we will end up with a much bigger trading base.
The economy will do just fine.....
Pete, where do you get this confidence from? TTIP, for instance, was over 10 years in the making before Trump squashed it, and there was much in that agreement that people were unhappy with (rightly or wrongly). Add to that Trump boasts at every opportunity that it's America first, so I'm not sure how we can really believe that a) a deal will be done quickly (as trade deals just aren't - ever), and b) that it will be better for us than the one Trump scrapped.
This is the kind of thing that so frustrates people, as it's a level of gungho optimism that has absolutely zero basis in historical fact. Yet despite such things never having happened at all in the past, people seem adamant that they will happen in the future with bells on.