These wrenches were sent over during the war and certainly date from that period.  I believe that they were part of the war effort and not intended for retail, although they have no government makings to show that they were armed forces issue.  I have several trade catalogues from the immediate post war period and have never seen Plomb tools listed in them.

The difference between BS and Whitworth sizes is that the head size of the bolt/nut is always one size larger for the BS spanner (e.g., 3/16 Whitworth = 1/4 BS).  Whitworth and BSF are 2 thread types; Whitworth is coarse and BSF is British Standard Fine.  Both thread angles are the same at 55 degrees, but the pitch is different.

Reed adds:

Plomb made a few Whitworth sized tools for the retail market right at the tail end of the Plomb name but there are no catalog listings until the first Proto catalog in 1949. The addition of British sizes to the Plomb line was due to the surge in popularity of British cars & motorcycles in America after WWII.

A visitor named James sent this comment:  

The wrench shown in the photograph on the "British Tools" page is actually for the "British Association" (more commonly called "BA") thread range, rather than BSF or BSW. Further information on the BA and other British thread types can be found at: http://www.britishfasteners.com/threads 

After reviewing all of the fastener types, Bob says:

With seven different types of British fasteners, no wonder somebody invented the Crescent wrench!