can I screw into a UPVC to fix a curtain pole?

Uvpc units, have thin metal box section inside it, for strength and rigidity, so you will need to drill into that as well, Do not drill on a joint, that is upvc only. The fixtures will probably be stonger than on conventional wood windows, so no problem hanging anything, use a self tapping type of screw, better grip and it will go in easier, and stay longer.
They do say anything like that decreases the insulation, but for a 1mm hole, I do not think it will reduce it to any extent that you will notice. If you need to remove the fittings, later on, use a upvc adhesive/small gap filler, that will then reseal the unit.

SD
November 2011