A legal framework already exists for zero, or near-zero, tariffs between the UK and EU post Brexit in case on no agreement on a EU’s favoured deal.
A helpful paper was produced for the European Parliament n May 2018 entitled The Institutional Consequences of a ‘Hard Brexit’. That paper cites article XXIV(3) and the “frontier traffic exception” of GATT (General Agreement on Tariffs & Trade) of the World Trade Organisation (WTO).
So given the GATT from a UK perspective panic about “crashing out” without a EU deal seems to be as wrong as the “project fear” put about before the 2016 referendum -or maybe just a continuation of the same.
Eamon Mc Nicholas