just to clarify: Do you mean that those numbers in my link are indeed tracking numbers for the UK?
The code below the barcode is indeed of the type used as a registration/tracking number.
And the destination country carrier assigns a new one?
PostNL certainly does so on occasions. But even this does not seem to be a certainty. My latest delivery from the UK had a code starting with RV. PostNL's tracking site uses 3S. Sometimes, I get an item with a PostNL sticker with a 3S barcode. This time, it was not the case. I have given up on understanding how this works.
You may be able to track the item using the original code on the Royal Mail site. However, the information may not be updated once the item has been transferred to the receiving service.
I do not get why it is said that these new numbers are no tracking numbers?
The link suggests the number is used for administrative purposes. Since you do not pay for a tracking service, that service is not provided.