[imtringued]

The technical argument is that the routers that are physically located in the UK are passing the packets through, not the website operator.

This is the same as letting a delivery cross your borders, except the delivery vehicle here is permanent infrastructure, similar to a pipeline and it is purposefully set to be permissive and allow anything through.

Why are you suddenly pretending that there is no equivalent to the customs office in this scenario?

It's not like the website operator is sneakily smuggling cargo on a container ship. VPN usage is done UK citizens. The operator has already denied shipments to UK addresses in this scenario.