Can I fit a valve with 15mm connections without replacing 22mm pipework?
Cant you find a 22mm-15mm adapter?
I live on the other side of the pond and to change sizes there are adapters both male and female to make the change.
I just googled a “22mm to 15mm pipe adapter” and a brass piece for $6.69 came up on the first page.
adapter
We use national pipe thread so I don’t know if that would work but try it.
It is a male thread so it should screw in and then female so your new piece would screw into that if the thread pitch is correct.