If it's rigid plastic then it's not Unipipe
. What colour is it, and what is written on the pipe? I suspect it is uPVC, ABS or similar, with solvent weld joints. Adaptors are available through the suppliers of such pipe (for industrial applications), but probably not plumbers merchants.
You could use an adaptor made by Yorkshire fittings
intended for 3/8" poly pipe x 15mm copper but which should work on 16mm OD plastic (same OD as 3/8" poly) if you don't over tighten the compression nut. I have a couple of imported 16mm compression fittings
for the purpose (it's a standard size in France, for example).
Another improvisation that I have used is an old style Hep2o push-fit tap
connector which will accommodate 16mm pipe if the retaining nut is reamed out to 16mm. The O ring, grab ring and socket will take 16mm OD without adaption.