Jack has provided this overview of the BPA's role in the UK's import and Export controls during and after WWII.
Initial Control Procedure
Heavyweight Mixed Franking Item to Australia
Processing Rate Drops
Very Early Incoming Item/Censorship Procedures Change
Licensing Requirements for Exports
The System in Action
Parallel Philatelic Control in the U.S.A./Mail to South Africa
Military Censorship Ends
Revised Approval Stamp at B.P.A.
Milestone – Count Rolls Over/Mail to Turkey
Another Processing Milestone/Mail to Chile
Treatment of FFCs/Why London W11 for Reg'n?/Mail to South America
Returning Unwanted Approvals to the U.K.
Another Change in Design of Approval Stamp/Introducing Type 3 Approval Stamp
The Final Days/Processing Data Graph