If there is a No-Deal Brexit the WTO comes into force. The UK will then have to give a confidence and concession schedule whereby they list the amount of goods they are prepared to import per year.
Then the rest of the members of WTO which about 164 countries will be given 3 months to object.
But countries which includes Brazil, Russia and Argentina, can put their objections to the UK’s confidence and concession schedule. Then they have to await for the WTO Adjudication for one year. In the meantime they could, if they wished, flood the UK’s economy with cheap food and cars.
Note that if the UK dropped all tariffs on EU made goods, it would need to do that on all imports from ALL other countries as per WTO non-discrimination rules
The UK would instantly lose negotiating leverage. It would then have no tariffs to negotiate away. While other countries would now enjoy tariff free access to UK markets, while continuing to protect their own markets.
The problem is the consequences. Once tariff-free imports of a product are available from places with far lower wages, quality standards, human rights and checks and balances it becomes very hard for the UK to compete on that product with all the extra costs UK standards and rights cost you.
As perfect example is Chinese Steel - a tariff increase was put forwards to protect European steel manufacturing, including ours.The UK vetoed it, then blamed the EU for the UK steel industry declining because it couldn’t compete.
Britain’s manufacturing industry is heavily high tech and not easily replicated in third world countries, but Britain does not produce many raw materials (even the iron ore and coal used to produce UK steel has to be imported now) this would reduce the cost of raw materials to UK industry. With the lowering of the pound, our manufacturers would suddenly find their products more competitive.