Although the point about it being the PP has been made, the story behind its introduction is possibly, typical of procurement.
The RAF only got them when a member of the DPG had a stoppage with one during an attempt to kidnap Princess Anne in London, back in the 70s. They decided to get rid of them, and only realised afterwards that they had made a blunder with servicing (I think). A small pin should protrude at the rear of the slide above the hammer indicating a cocking action, but it was reported that this occasionaly broke, causing possible feeding problems. I think I'm right in saying that the officer's PP wasn't modified.
Not a massively effective wpn across the baord, but for close and immediate protection, more than adequate. The half cock style capability especially, and light action, being ideal if you've been injured in the descent.
http://en.wikipedia.org/wiki/Anne,_P...Kidnap_attempt
http://news.bbc.co.uk/1/hi/uk/4139187.stm
http://www.ianball.me.uk/