The Skilling ceiling you describe are not that easy to insulate from the loft. The plaster board/lath ceiling is normally tight up against the rafters, giving you the needed height in the room. Stuffing in fibre glass is an option but you will stop the air flow between ceiling and roof tiles, causing no end of trouble with condensation. The best method is some sort of metal foil insulating barrier or insulated backed plasterboard. All good when building from scratch, but not so easy when fitting retrospectively. Taking the plaster lath or plasterboard ceiling down is messy and the skill involved is the plasterer making good the new to old join.