The EU is the UK's largest trading partner, with around half of all UK exports and imports going to or coming from the Single Market. EU economies are highly developed, so our trade with the EU focuses less on labour intensive goods and services, than that with developing economies, and focuses more on financial services, insurance, and other services focused exports. Essentially, a significant curtailment of trade with Europe would force the UK to shift away from producing the things it's been "relatively good at", and therefore tends to export to the EU, and towards the things it currently imports and is "relatively less good at". When the UK leaves the Single Market, it will need to negotiate a wide-ranging trade deal with the 27 EU Member States, which is something that could take years. Moving away from exports like financial services and insurance that the UK was providing to the EEA, and towards exports like raw materials and food provided for less economically developed countries, would have a detrimental impact on the general economic welfare of the average Brit and the economy as a whole.