FWIW, I found that the combination of a 50mm deep service space behind the plasterboard, plus the space allowed behind units, the bath etc, was such that I could easily just sweep the plastic pipe around into a gentle bend to come out perpendicular to the wall by the time it needed to go through something like the back of a cabinet. I then transitioned from plastic to copper using plastic push fit valves, that when in things like the cabinets that house the washbasins, sinks etc are pretty much flush with the back of the cabinet interior, so you don't see the plastic pipe. It did mean having slots in the plasterboard behind units where the plastic pipes came out, but these are hidden from sight and the guys doing the plastering didn't have any problem just cutting slots to allow the pipes to poke out. I'd left the plastic pipes too long, and taped over the ends with red or blue electrical tape, to keep the muck out and show which was hot and cold.