1/2" BSP and NPT threads are the same thread pitch and nominal diameters (there are minor differences in some of the other parameters - profile and included angle). A NPT male fitting will generally seal perfectly well in a plastic BSP female fitting and vice versa without an adapter, although it's probably not the best practice, particularly if there's any serious pressure involved, or you'll be remaking the connection often. The same thing is done with the cheap 12/24V DC pumps with plastic 1/2" fittings. This doesn't necessarily work at all with other nominal BSP/NPT sizes though, or with stainless threads on both sides.