china and usa dont have access to the EU
single market. they do have trade arrangements with the EU, thats it.
This will be of course a option for the UK, but it will take some time until such trade-deal is done. And even if the EU and the UK make a trade arrangement, then the UK will be treaten same as the other non-EU partners; means that goods from the UK will be controlled at border, and maybe even a trade-tax be put on the goods.
But the worst part will be that it will all take longer until the goods arrive at their final destination.
For example: Lets say the UK is selling 100 tons of meat to the EU. The meat will be put in cooling containers so it dont spoils, than brought to a Europen harbor (Rotterdam for example). And then it will maybe take 2-3 extra days until the EU inspectors give the green light. (maybe even longer?)
So lets say in our easy example the UK is selling their meat for 1000€ per ton, but because the shipment takes now 2days+ longer the price increases to 1100€ per ton.
And now lets also put a 10% tax on non-EU goods, so the ton will cost you now 1210€ per ton.
And who do you think will pay the diffrence now? the customer of course. But not every customer will accept it. Which means that the UK will sell at the end less goods, make less money, people will lose their jobs and so on.
Same thing will happen on the EU side of course. The EU will sell less goods to the UK, will make less money, lose jobs etc.