Timothy Murphy brought next idea :

Is the plastic pipe likely to be glued into the copper pipe?
If so, do I need to cut through it?

It looks to my eyes as if a male to female elbow has been used, so the only
way to unscrew the elbow, is to first release the nut between copper pipe
and elbow, then unscrew the entire elbow from the plastic thread.


Or release the outer plastic nut a little then rotate the whole valve
(removed the float arm first) to unscrew it from both outer nut and brass
elbow. Remembering to turn off the water supply first!
I would then replace that elbow with a service valve 90 degree tap
connector.

Mike