Britain has two postal systems. The main one is the well known Royal Mail. The vast majority of mail goes through this system and uses the standard UK postcode system. If you have sent mail to or from Britain, this is almost certainly the system you have used. The other system is the British Forces Post Office (BFPO). It is for sending post to British Forces abroad. Technically this means it predates the Royal Mail, as the first English postal system was set up for English soldiers in Scotland or France. The idea is that you send mail to the BFPO address in Britain and then the government will ensure it reaches the appropriate person around the world without handing it over to anyone outside the British government. Normally this means the military delivers it, but it can also go through diplomatic or foreign office channels.
BFPO address look quite different to Royal Mail address. They just have the addressee’s name, their id code and the BFPO Code. Sometimes a postcode is appended for getting through some mail systems, HA4 6EP - check it out. For example BFPO 792 is Camp Bastion in Afghanistan, BFPO 305 is the navy ship HMS Illustrious and BFPO 5439 is the High Commission in Kuala Lumpur. That last one is my new address and thus I have discovered the great problem with BFPO addresses. Very few people seem to know how they work. To all UK (and to a lesser extent, worldwide) web developers, there is more than one UK post system - please handle it! In particular, pooks to you HSBC - I expected better from an “international” bank.