The MFN clause forbids countries from offering preferential trade terms to another country unless these terms are offered to all other members.
At present the UK enjoys such an exception + preferential trade with many countries by being in the EU.
With no FTAs or CU deal, the UK is no longer exempt from the MFN clause = it must give all countries the same tariffs. The choice is raising tariffs on EU goods or lowering them on goods from elsewhere.
i) Prices fall making UK farming + fishing unprofitable
ii) Incentives for trade deals disappear. Why should other countries give the UK access to their markets if they already access the UK?
As setting up new trade deals takes years, it would be stuck in that position for years.
1) it has the pressure to make deals very quickly,
2) it has losts its stake in the EU and will be negotiating for the return of benefits from the outside,
3) it has lost good will of the EU27.