Valve length

[Ajax Bay]

Why doesn't it have French sizes listed and why did you write it in a size I don't know about?
Halford's website says the tires are 700 x 42C.

ETRTO designated size description not "French". Halfords are pandering to the incognoscenti.
Inform yourself and banish confusion by skim reading the first and accessing more knowledge than most will need thanks to the late Sheldon Brown:
https://www.evanscycles.com/buying-guides/inner-tubes#innerTubeSize
https://sheldonbrown.com/tire-sizing.html
You might find Relja more consumable: https://bike.bikegremlin.com/285/bicycle-tyre-dimensions/