Very interesting.
Looking into these BSF (British Standard Fine) threads I see they have a different thread shape when compared to the standard threads used in the USA and in the Metric thread system.
The included angle from one thread surface to the other side of the V is 55° rather than the 60° angle we commonly use for the Unified threads. The British Whitworth threads also use this 55° included angle. This different thread form, even if the sizes of the BSF threads were the same as our Unified threads (which they aren't), would prevent a Unified thread from screwing into it easily if at all.
The external 1/4 BSF thead has a outside (major) diameter of .250 with a thread pitch of 26 threads per inch.
The external 9/32 BSF thread has a outside (major) diameter of .2814 with a thread pitch of 26 threads per inch.
There are no standard Unified threads that use a 26 threads per inch pitch and there are no standard Unified threads with a 9/32 major diameter. Even if there were, the thread shape would prevent the 60° angle of the Unified threads from screwing into a BSF threaded hole.
I don't know of any US distributor that sells nipples with a BSF thread series. Are there any makers of these in the UK?