We have a loft room that is boarded and has insulated plasterboard attached to the rafters. There is a small gap between the bottom of the plasterboard and the flooring which I assume is compromising the insulation. I understand it is important not to block the space behind the plasterboard as this is needed to ventilate from the eaves, so I am reluctant to use expanding foam to fill this. What do people recommend?
The best course of action would be to remove the plasterboard (as it seems to be improperly installed, horizontal, and unfinished) and tightly insulate atop the studs and therefore behind the plasterboard. Large sheet/panel Styrofoam is the best very long-term option.
However, if you're mostly concerned about aesthetics, then just a tightly fitted and caulked baseboard would fit the bill perfectly.
Eave ventilation is only important with a proper path to Gable Vents, behind the walls, or to a Ridge-Vent system with a proper path behind the ceiling only.