harryagain wrote:

In general in the UK all pipework [...] is sized by the nominal
bore. mm comes from the EUSSR frogs

The frogs do tend to use myriad different sizes, e.g. for copper elbows
compare and contrast ...

http://toolstation.fr/shop/d110/sd2698/p39317
http://toolstation.com/shop/d20/sd2698/p86114

Surprised their unions haven't found a way to outlaw push-fit, try to
buy a "pince à emboiture" over here, they don't seem to make a 15mm
"matrix" for them ...

http://youtu.be/__X3K2EmPCQ