Royal Mail Group plc is the parent company of Royal Mail, Post Office, and Parcelforce Worldwide. All are apparently licensed by something called Postcomm after the 2000 Postal Services Act. At least as fo mid-2005, Royal Mail lost a bid to ban the posting of guns through the mails per Postcomm. Essentially, the Royal Mail grew alarmed at the number of guns it found it was delivering because all mail going by air was screened to reveal contents. I was not able to find any more current information that would suggest a change.
This worked through July of this year based on my experience.
