What happens if Brexit negotiations don't work?

Press quote (The Atlantic)
John Springford, Simon Tilford
12 September 2017

As noted by John Springford and Simon Tilford of the Centre for European Reform, a cliff edge scenario - in which the UK would leave the EU (including its single market and customs union) without a trade deal or transition period - would make the 40 percent of overall UK exports that go to the EU subject to automatic tariffs and customs barriers.