common home plumbing adapter sizes explained (I think)

I talk : BSP adapter fittings for the home (bathroom, kitchen etc).
example : 3/8 inch BSP tee (male male female)
update : I now think the middle 1 is 1/4.
Never been as confused as trying to work out how to be a piping tee that fits.
Every form of measurement or identity is an illusion.
I bought a bidet (euro size apparently .. 1/3 inch bsp).
Didn't fit my UK toilet (1/2 inch bsp).
Bought adapters and of course got it wrong (3/4 inch bsp).
How is this situation allowed to go on ? Switch it all to metric.
Screwfix do reducer bush at £1.53 ?
Ebay will be like £3+.
Finding it hard to find an 'increaser' bush.
Also the names. Searching on ebay is a nightmare as who knows the names.
Use 1 name you rule out loads in the search.
I'm calling this a 'tee'.
Would be nice for ebay if they used the amazon locker idea.
Issues :
BSP (british tandard piping) inches is not the same as IMPERIAL inches.
Plumbers say 1 inch BSP = 25mm even tho its like 33.5mm.
theres BSP and BSPP(parallel). I'm guessing home stuff is BSP ? (tapered).
Most of the things have archaic names (e.g. bush, tee)
Male : thread on outside ?
Female : thread on inside ?
(possible system theory e.g. stalagtytes).
measurements are from where thread is ? i.e. not always the external size.
chaos.
And imho switch to PUSHFIT type plumbing.
reference :
the archaic common BSP sizes for home ? (I gave up on metric, even tho as PM I would make it the only type)
1/4 (tiny)
3/8
1/2
3/4
1