Given that the central panel of my upvc door in set forward of the outer part of the door, is it feasible to glue an insulation panel to the centre panel to reduce heat loss?
If you examine the thermal conductivity of items used in any insulation system you will find the effective bit is an air gap. Now air could be udes in trapped expanded polystyrene. Sticking anything onto a door panel will detract from the appearance of the door.
Suggest removing any glass panel which will then give acces to door interior - add the polystyreme as small cut up pieces but make sure door handle and lock is protected from ingestion and stop working.
Alternatively make use of expending foam.
