„WTO rules“ are just a framework for trading rules. Those rules itself still need to be negotiated.

For example, „which form do you have to fill with which information in how many copies, in order to export pastry?“.

That‘s much faster than a real trade deal, because it‘s mostly administrative, but it is still a hurdle to clear before you can trade under „WTO rules“.

And now do that for a gazillion sectors and special cases. With every trading partner.

WTO deals can take years to negotiate. The U.K's land of heaven is lobbying in Brussels so they can get a fair to everyone deal. As my British friends say, can't have your cake and eat it.