Per Ardua ad Astra - ‘Through Adversity to the Stars’, The Latin motto of the RAF

Abbreviations
KIA - Killed In Action
MIA - Missing In Action
WIA - Wounded in Action
POW - Prisoner Of War

Abbreviations found in National Archives file AIR 2/6876 [*]
u/t - under training
AOS - Air Observer School
ATS - Air Training Scheme
BGS - Bombing and Gunnery School
EFTS - Elementary Flying Training School
GRS - General Reconaissance School
ITS - Initial Training School
OTU - Operational Training Unit
SFTS - Service Flying Training School
WS - Wireless School
PD or PDC - Pilot Disposal Center
PRC or RC - Personnel Reception Center
ACH/GD - Air Craft Hand/General Duties

More abbreviatons and acronyms on Words of WW2 and RAF abbreviations

Ranks
AM - Air Marshall
AVM - Air Vice Marshall
AC - Air Commodore (Modern RAF)
G/C - Group Captain
W/C - Wing Commander
S/L - Squadron Leader
F/L - Flight Lieutenant
F/O - Flying Officer
W/O - Warrant Officer
P/O - Pilot Officer
F/S - (Flight) Sergeant

Decorations
CBE - Commander in The Most Excellent Order of the British Empire
OBE - Officer in The Most Excellent Order of the British Empire
BEM - British Empire Medal
MBE - Member of The Most Excellent Order of the British Empire
DCM- Distinguished Conduct Medal
DFC - Distinguished Flying Cross
DFM - Distinguished Flying Medal
DSO - Distinguished Service Order
AFM - Air Force Medal

Sources
AD - Audrey Dewjee
AE - Audrey Elcombe
CG - Cy Grant, A Member of the RAF of Indeterminable Race, War Memoir
CMHA - Chuagaramas Military History & Aerospace Museum, Trinidad
CWGC - Commonwealth War Graves Commission
MOD - Ministry of Defence
NA - National Archives [*]
TDA - The Daily Argosy, Guyana (courtesy Mr. J.A. Lee, CMHA)
UC - Ulric Cross

[*] the file AIR 2/6876 gives a list of 298 British West Indians (including 50 officers) who attested in Canada for training as aircrew, and a nominal roll of 340 personnel (including 47 officers) from the Empire including the Caribbean (courtesy Guy Grannum, NA)