I have a UPV door and its purrfectly able to have a cat flap in them. If you have a glass pane in the door at the top section, then its this that is airtight and not the UPVC below. Its not the easiest job in the worlf and can and IS messy. You need a template, then I would suggest drilling four holes - one at each corner and then cutting inbetween - possibly with a jigsaw. Then when everything fits, some silicone sealant to seal the gap between the outer faciure of the cat flap - sort of like the border thing and some galvanised screws - has to be galvanised as otehrwise will rust in the rain and thats a pig of a color to get off white UPVC. Then its a case of letting it set and away you go.
If I was there I would do it for you happily for a coffee or four!