The main difference between a British Standard Pipe Taper and a National Pipe Taper threads is that the NPT thread have a 60-degree included angle compared to the 55-degree included angle on the BSPT thread. NPT threads also have flattened peaks and valleys called a Sellers form, while the BSPT have rounded peaks and valleys called the Whitworth form.
The NPT seal is the most widely used seal in the United States and Canada for pressure calibration. The tapered male NPT thread wedges into the female NPT adapter in such a way that as the seal is tightened, the force generated between the fittings can handle pressure.
The BSPT on the other hand is the most commonly used thread in other countries. A thread sealant will be required in order to seal a BSPT connection together properly.