I used to work seasonally for Royal Mail at a sorting office. Many compaies (icluding DWP) outsource their mail to computerised companies. This mail is then run through a Postcode reading machine which checks the amount of postage paid against the destination and, if they do not match rejects the item. At the end of the shift these rejects are collected, sorted and returned to sender, Many of the company computers were not programmed to put any more than UK 2nd class postage on all and could not distinguish the need for extra postage on overseas items. What happened to those returns, I do not know, but I would be surprised to find that there was any mechanism to send them out again with the correct postege.