@BackyardBullion the guy has seen that precious metals are prohibited items. That does not mean they won't take them and 99% of the time the mail gets through, it is just the courier will wash their hands of a lost package. The list of prohibited items is very long for the lower prices couriers.
I am not sure what the dealers are doing - i was explicitly told by a German dealer using DHL where i had absent mindedly put my old postcode down and the package was not delivered but got returned to Germany to be sent again (with the correct postcode) that i did not have to worry as everything was fully insured. i think it was an order for 700 x 1oz Panda coins, so it was not something anyone would be happy to go astray. A dealer shipping £millions of coins is not going to take the risk.
It might be useful you asking Lizzie from Chard (a member of the forum) what they do when they send items outside the UK.