Should be BSPT really, which as you have all said stands for British Standard Pipe Thread, sized are 1/2", 3/4,1,1.1/4,1.1/2, 2" etc.
The metric equivalents are now being used 15,20,25,32,40,50mm a bit more.
Anything with threads is in imperial sizes eg 1/4, 3/8, 1/2, 3/4 and so on and comes from, as Manchester gas said, the internal diameter of the relevant pipe.
In the UK we know them as BSP threads. British Standard Pipe. This thread comes in 2 types. BSPP (parallel) and BSPT (tapered).
This thread sizing also applies to European ISO standards where BSPP is known as G eg. G 3/4 and BSPT is R eg. R 3/4.
The thread on the plug and airvent are 1/2" BSPP or G 1/2
From what I can see there are 4 different BSP threads, one is parallel two are tapered with either a 55 or 60 degree taper but then there is another called BSPF which I think might be gas related.