I think that the risk to PVC cable sheathing from insulation is more related to the plasticiser in the PVC interacting with polystyrene insulation - that looks like polyeurethane foam, so probably not a big problem from that point of view. For maintainablility, I'd refrain from putting in any more foam above the CU where the cables go out to the individual circuits.
You seem to have a 'spare' circuit breaker module sitting on top of the CU - I'd get rid of that. If you've got draughts coming from behind the plasterboard, I'd try and stop them at source - wherever that is.
I think I'd simply try and add a foam draught strip all the way round the edge of the cover panel - to make that airtight - this may involve adding a small strip of wood across the bottom of the opening. You'll still lose heat because air can clearly circulate behind the plasterboard.
The electrical regs have been tightented up recently with regard to fire risk around the CU - CU's now need to be metal and cable entries caulked/airtight to contain any fire starting within. Because of the high currents flowing, and the use of screw-connectors (which can work themselves loose over time) fires have started in or near CU's - I found charring in mine caused by exactly this. So - you do need to be mindful of fire-proofing or at least the possibility of overheating being a risk, but also accessibility for future maintenance/testing near, your CU.
Cheers, W2S
