My plumber used plastic Hep20 fittings with copper where you could see it, but to be honest I would have been happy with plastic where you could see it as I now have to find a way to make the copper pipes white either with paint or covers.
You don’t have to buy the plastic in coils, straight lengths are available you could use to the rads.
Hep20 fittings don’t work with chromer, they do seal but they don’t grip the shiny surface (they need to dig in) so as soon as pressure comes through the pipes they pop off. Ask me how I know this…….. 🙁
I’m sold on Hep20 though, love it, never get any problems. It isn’t cheap but its much better than the JG Speedfit stuff I used to use.