Hi there,  
I would like to install a mixer shower and was wondering if i could use plastic pipe due to it being easier to use?
I wanted to know that if i was to use the plastic inner sleeves to reinforce the pipe lining under the compression fitting and use the  olive's made of copper which are a little softer inside the compression fittings would this be sufficient and ok for the mixer valve?  
The boiler that is feeding the hot water is a combi if this helps, i think this is a high pressure boiler.

Make sure the pipework is well clipped as well bud, been to a couple with plasic pipes that were not and where pipe slapping against stud wall when valve shut off quickly.