Remember that nominal pipe sizes , both US and British, do not match the actual size of the fitting. The Nominal is always smaller than the actual size of the fitting. the nominal size is related to the internal size of the pipe that is being used.
A 1/8 inch NPT is closer to 1/4 inch OD but the pipe id is closer to 1/8.
Also, I understand that BSP stands for British Straight Pipe, as opposed to a tapered fitting as in NPT, National Pipe tapered