This is an interesting map showing what trade agreements the EU has around the world and those that are still in negotiation (click to enlarge):
The approximate quantity of trade that Britain conducts with the EU and with EU negotiated trade agreement countries is:
EU 44%, USA 13%, Switzerland 5%, Hong Kong 2%, Turkey 2%, Japan 2%, South Korea 2%.
That is getting on for three quarters (70%) of UK trade which will be slower, more inconvenient and probably more expensive.
For a trading nation that cannot feed itself, voluntarily opting to make such a high proportion of your current trading more difficult is – there is no other word – insane.