No-one can predict what will happen until we know what kind EU deal we are going for. If we go for a hard Brexit, quite apart from the problem of renegotiating our entire trade portfolio + new trade deals, we will have to deal with loss of the single market membership/customs union.
The EU still accounts for 45% of our exports (by comparision, the US is 16.6% and China 4.5%.)
Sir Andrew Cahn, fomer chief executive of UK Trade and Investment, stated that it was 'completely unrealistic' to think that the UK could negotiate free trade agreements with other countries to make up for lost access to the single market. 'At the very best the very best of the promised network of trade agreements would not be remotely on the scale of current trade through the single market'.
He warned that deals with US and China would take at least a decade. And pointed out that the US has not ratified a single trade deal in the last 5 years, and that China would extract a high price for access to its markets.
Hosuk Lee-Makiyama, director of ECIPE, gave evidence to a Treasury Select Committee:
'Unfortunately, the realities are such that in a world where the UK trades 50:50 with the EU and the rest of the world, whatever trade we lose with Europe cannot be recreated with the rest of the world'.
'It is just not in the balance. The market access we would obtain from the free trade agreements would not equal the trade loss we would have with Europe. It is just a different scale. All in all, we need to do both'.