From Debretts:
Retired and Former Officers
Marshals of the Royal Air Force remain on the active list for life, and so continue to use this rank.
Other officers of the rank of squadron leader and above may use, and be addressed by, their rank after being placed on the retired list.
It is official practice for the Ministry of Defence to add the abbreviation ‘retd’ after a retired officer’s name, although this is not strictly necessary.
Not looked at QRs yet, but I think Debretts draw their information from there. Don't know if this applies to commissions resigned or not...