I think the crux of this beef is that @jellybaby is saying the China>UK customs checks is already a well established system and the new EU>UK checks are additional to this so they shouldn't be affected, @sumo is saying that the additional new EU>UK checks are being taken on, at least in part by the systems involved in the China>UK checks, and so are being affected, but neither has seen the subtle difference in the others point of view. As for who is right I don't know.
I think the crux of this beef is that @jellybaby is saying the China>UK customs checks is already a well established system and the new EU>UK checks are additional to this so they shouldn't be affected, @sumo is saying that the additional new EU>UK checks are being taken on, at least in part by the systems involved in the China>UK checks, and so are being affected, but neither has seen the subtle difference in the others point of view. As for who is right I don't know.