As an amusing aside, there isn t a British Standard covering British Standard Brass . Technically there is no such thing as BSB, though everyone calls it that, ho ho.
26 tpi Whitworth was widely used in the UK on brass tube because 26tpi BSW is well suited to thin walled pipe. It s a trade application of an existing thread, and was originally called British Brass Thread .
I guess chaps in the past assumed brass thread was as formally defined as BSW & BSF, and promoted it to British Standard Brass by common usage. And we still refer to BA despite this genuinely being a British Standard thread.