This info came from Pete Verdon via email
Mill Hill was comprised of two parts at the same location. There was Inglis Barracks where the barracks, NAAFI and training school were located. Then there was the sorting Depot itself, at the bottom of the hill on Frith Lane (NW7). Both in Mill Hill.
As for the names, the order was as follows:
HPCCD up to 79
PCD RE from 80
BFPO from 1999
All of these names relate to Mill Hill, except BFPO, which applied to Mill Hill from 1999-2007, then to RAF Northolt from 2007-present. Hope this makes sense? As far as the unit listing is concerned, I think it would be best to leave PCD as it is. The short history section should explain things, together with the Mill Hill link
This was THE main HQ for the BFPO postal service for HM Armed Forces around the world (including HM Ships/RN) from 1962. This was the postie depot unit, so all posties trained here too (B3, B2 &B1). I was there between 1978-1981. HPCCD changed to HPD in 1979 (Home Postal Depot). The barracks was bombed by the IRA in 1988, killing one soldier and injuring 9 others. In 1999 became it became known as the British Forces Post Office. 2007 BFPO moved to it's present location at RAF Northolt. Hope you find this information useful,as I'm sure a number of sappers were stationed there, certainly all the posties at some point or other!
Pete (Joe90) Verdon