Here is some information about the various British Field Post Offices,most of whom would be issuing postal orders

Simplified lists;

Army & Royal Air Force; http://www.bfpo.org.uk/bfponumbers_static.htm .

Royal Navy; http://www.bfpo.org.uk/bfponumbers_ships.htm .

Here is a specialised list;

https://www.superletter.com/bfpo/html/index.cfm .

Aidan.