Perhaps strictly a tangent but it's clearly related. You describe the UK as a net contributor. That is in reference to the EU budget, which has nothing to do with trade. We also run a trade deficit, it is true, but the phrase "net contributor" refers to the EU budget, as shown by your link.
Nobody is arguing that the EU and UK will stop trading, either. What is at issue is the smoothness of that trade. A No Deal exit, by definition, disrupts that smoothness, by reducing the volume of possible trade (owing to increased requirement for paperwork, etc), and by changing prices (owing to tariffs).