BSP stands for ‘British Standard Pipe’ NOT the same as GAS (slightly different). In BSP the size is the diameter of the pipe not the overall diameter.
Other types are:
BSW- British Standard Whitworth
BSF- British Standard Fine
UNF- Unified National Fine
UNC- Unified National Coarse
BA- British Association
BSC- British Standard Cycle (some just marked ‘Cycle’).
BSB- British Standard Brass
Taps and dies is BSP are common, if you do not know where from or are unable to obtain them PM me as my father sells them!! He can also measure external threads so you can get the correct size tap/die.