Draught proofing under a Switchboard

Expanding foam should be okay, just give the small print a read on the back of the can first. It’s expanded polystyrene that causes an issue with PVC cables due to plasticiser migration. You could also use a fire rated expanding foam whilst you’re at it to stop the spread of fire as a Brucie Bonus.

Yes use fire foam available at screwfix or toolstation it's just like ordinary expanding foam but fire rated

Fire rated expanding foam. BS 7671:2018 Regulation 527.2 sets out the requirements for the sealing of wiring system penetrations, Regulation 527.2.1 states that: “Where a wiring system passes through elements of building construction such as floors, walls, roofs, ceilings, partitions or cavity barriers, the openings remaining after passage of the wiring system shall be sealed according to the degree of fire-resistance (if any) prescribed for the respective element of building construction before penetration.”

Personally I'd be carefull using expandable foam as I fear over a long period of time the insulation on the cables could detoriate. I'd fill with glass wool and top it of with painters caulk or plasters finish. Hope this helps