After years of messing with these beasts, I have a collection of mongrel taps and dies in BSW( Brit Standard Whitworth), BSF(Brit. Std. Fine), BSC( Brit Std. Cycle). Also found on the bike are BSP( Brit. Std. Pipe), BA( Brit. Assoc.) machine screws, and some weird thread on speedo/tach drive cables that I think is British Standard Brass, 1/2" x 26tpi. Find a thread gauge and it will be very helpful. Commandos had every known thread pattern and it will drive one nuts, American, British, and metric.
Download this, very helpful: