The plastic threaded pipe on a replacement cistern float valve does not have the same internal diameter dimensions as a normal pipe and measureing the outside diameter of the plastic pipe has no relation to the bsp thread size.
So, unless you work with these pipe threads regularly, there's no obvious thread size.
Recently i threaded some 2" metal pipework. The internal diameter was 2", outside OD was approx 2.5". Thread size 2"bsp.
But in normal engineering, the thread sizes are measured on the outside diameter M4,5,6,8,10,12,14,16 etc all metric threads are outside diameter.
So 1/2" bsp is the same thread size on a cistern supply and on a 15mm compression joint(copper) ball valve?