Now, just to throw a spanner in the works.......
I use plastic pipe and fittings a lot. On 15mm I use PolyPlumb and on 22mm I use Speedfit or PolyPlumb.
The PolyPlumb pipe inserts are stainless steel so very thin and have no effect on water flow. The Speedfit inserts are plastic and quite chunky but I like the ability to 'undo' the Speedfit fittings.
Speedfit have changed the design of the fittings and now you can lock them shut by twisting the 'nut' on the fitting and you can even fit a collar between the nut and the end as added security if you want. Old Speedfit fittings could come apart if the collar was pushed against something (a stud frame for example).
I've never had a fitting fail although I had one leak once. This was due to the plastic pipe being badly scratched where we dragged it through a hole drilled in a wall. The O ring couldn't make a perfect seal on the scored pipe.
I like the way plastic pipe fittings can rotate so avoiding the twisting torque you get with compression fittings. They are also extremely quick to fit.
Plastic pipe also eliminates the need to supplementary bond the pipework in a bathroom. I always use copper for the last bit up to the rad or towel rail (just for appearance sake).
The 22mm Speedfit I now buy in 3m straight lengths as the 50m coils do NOT want to straighten out!
So I now buy much less copper than I used to. Plastic pipe is more resistant to freezing as it will not split (we all know copper will) although this does not mean you don't need to lag it!
Never mix 'n match makes. If you want to join Speedfit to PolyPlumb or to Hep2o then use a short length of copper between as they are all compatable with copper. Alternatively use a brass compression fitting to join them.