What they do not get is that trade in the EU is not negotiated with a single government. Every member gets to veto a trade deal which prevents the entire 28 member states from acting in their own self-interest. So Britain cannot cut a trade deal with the USA or China without EU approval. Then the speculation goes that they will have to pay dues with no vote.
If true, all I can say is - wow. Germany and France control all trade deals made by Britain?