Ugh, what a situation.
honestly I'd just use pushfit with flex pipe (always the barrier type). Inserts on every joint.
the OD is the same as copper. You can use plastic pushfit onto copper . . .
Installed hot water plus central heating on the boat using this (boiler, hot water tank, mixer taps, radiators, bath). Only problem I had was when I didn't drain it down over winter, severe cold froze the water and pushed a joint apart. Very easy to fix.