Brass 1/2 inch low pressure BS1212 part 1 ball-cock.
The internal components of this valve are larger than those in the high pressure version, to allow a better flow of water.
Threaded ball mounting end has 5/16 inch British Standard Whitworth (BSW) thread - use standard 4-1/2 or 6 inch float.
Normally used in gravity (tank) fed applications, such as older toilet cisterns.
BSP (British Standard Pipe Thread) sizes don’t match up with what you would measure using a ruler. Have a look at the BSPT link at the bottom of this page for more information about how plumbing threads are measured.