I notice you have not had a reply since your last post. I will try to give some of the benefits of my experience re plastic pipe.
BUT, I must state that I am NOT a plumber.!
I have used the plastic tubing several times in the past to great effect. The best stuff I have found is 'HEP 2' (Grey in colour) it's very much more flexible and easier to handle than the DIY type white tubing. I noticed this weekend, that it was now available from
Wickes.
You can by this stuff on a reel, so several meters can achieved without a join. A great advantage when feeding under floorboards or down walls. There are also some very good 'pushfit' connectors available too. Some have a fairly large collar on, and can be easily removed. Others are much slimmer in profile, but once fitted, cannot be removed. I used copper pushfit connectors on mine, to keep the joints as slim as possible.
One thing that should be noted.... The plastic tubing or pushfit plastic connectors, will break the 'Earth bonding continuity. As far as I'm aware, all the pipework in a house should be earthed. Theoretically, you may have to run a few earth wires about, to ensure all is safe.... (I'm not a sparks either, and would recommend seeking proffesional advice on this...)
As far as joining copper to plastic, the pushfit connectors will fit both tubing. So you just push on the copper one side and the plastic on the other. You have to use an insert in the end of the plastic tube, to prevent it from distorting when hot. I have never had a leak with these connectors.
It is also possible to use compression fittings, but it never feels quite right when tightening on to plastic, and I don't like it.!!
It's best to cut the plastic tubing with a purpose made cutter, to ensure the ends are truly square. I found a cheapy hand held type (Like garden secateurs) at 'The Plumb Center'
Hope this helps out a bit.
Good luck with your project.
Flashbang.