Hi Chris & welcome to the forum.

The dates with the BFPOs mean a lot. I believe they are used similar to ours here in Canada. Whatever military units are attached to this BFPO means a lot too because of the dfferent military exercises and campaigns. For any given period of time that BFPO may have been located at location "Y" and after that it may have moved to another theatre and be located at location "Z". These are just hypothetical examples really. In other words, the locations for a lot of the BFPOs are static. There are of course, some permanently based BFPOs that do not move.
I too am having a hard time getting information of older FPO locations for Britain.
Chimo
Bujutsu