Correction (to almost everything): the £2K quote was for replacing both front and back doors (and frames, and putting in 5-point locks). Today's quote was £1790. (He seemed very impressed with today's quoter, but since it's a local firm in Reading, not much use to you I imagine.)

RPG doors are in fact GRP doors (a form of composite, Glass-Reinforced PolySomething). They're more wood-looking than uPVC, but better-insulated than wood.