Well selected. Other reviewers give varying figures. Even so, that covers Rotterdam, which gives its name to the effect but it is by no means the only port involved in this trade.
Antwerp is named, but even Algeciras in Spain is now involved in transshipment of non EU traffic to/from the UK. No one tries to quantify this traffic because it doesn't suit the establishment narrative.
Oh for goodness sake, do you really need to bring these silly conspiracy theories into everything? In the first place, the reason why this phonemenon has been given the name, The Rotterdam effect (or sometimes, the Rotterdam-Antwerp effect), is because those are far the most important, largest, ports that may effect those statistics, and it's generally believed that any contribution from other ports is going to be a lot smaller than any impact from the Dutch ports, and so isn't likely to have a significant impact on the statistics. In the second place, there have been attempts to estimate non-EU traffic routed via the EU. For example, the Office for National Statistics has
looked at it , as has HMRC. From some checking around, although there are some slightly different estimates, noone seriously thinks that it brings our EU trade down below about 40%, which is pretty close to the figure
trash80 cited. As for why there are no exact figures: You can't just wave a magic wand and get perfect statistics on everything... collecting decent data about something does usually take time and money, and until the arguments about Brexit came up, there was probably not much reason for the Government to spend the money collecting statistics about the ultimate destination of goods shipped to Europe. To get those statistics you'd have to start quizzing importers and exporters about what they are doing with their goods, which is potentially a lot more work than simply counting how much goes across the Channel, so it's probably not too surprising that really accurate statistics for that don't exist
It may surprise you to learn that the entire UK Government is actually not a big conspiracy whose sole purpose is to hide any statistics that don't conform to
furnessvale's viewpoint. They do have other things to do, like running the country.