The BBC’s Reality Check today says this:
The claim: The UK trades with “the rest of the world” (non-EU countries) under World Trade Organization (WTO) rules.
Reality Check verdict: This is wrong. With regard to tariffs, the UK trades with 24 countries and territories under WTO rules alone. With 68 others it has, as part of the EU, free trade agreements, either fully or partly in place, which all enable trade on better terms.
If you check here you will see that the list of 24 countries with which we trade on WTO rules includes our biggest non-EU trading partners, the USA, China, UAE, Hong Kong etc. Adding these up, they account for 32% of our global exports, which is 57% of our non-EU exports. So more than half our non-EU trade is conducted under WTO rules. (I don’t know why India with 1.1% of UK trade is not included here, since the EUs trade negotiations with India are currently stalled).
In fact much of our trade with the other 68 states mentioned is also conducted under WTO rules. The EU lists 50 FTAs, but on closer examination only a handful of these are fully comprehensive FTAs (see here and here) leaving much trade conducted under WTO rules. It would be complicated to calculate exactly how much, but it is clear the UK trades with most of the rest of the world on WTO rules.