Draughty walls

No, expanding foam is a filler, not a sealer.

You need to apply air tight tape to the floor to wall junction and refit the skirting. Make sure the surfaces are properly prepped etc (which may involve using a primer or air tight mastic depending on the substrate).

Doing this will have a big impact.

At the end of the day the expanding foam might be the right solution if you're looking to fill a gap and reduce the air-flow into the livable space and you don't have the resources or finance to address the whole solution. I don't disagree with MicktheMan at all, but air-tight tape isn't the first thing that I'd reach for if I was trying to stop a draft from a skirting board. Basically the drylined wall should have been air-sealed at the point of install.

Use a low-expanding foam, otherwise you may encounter other issues.

cheers - I note CharlieDIYte has a video where he uses this product to fill the gaps under his skirting boards. Might be worth a try, I'm not keen on taking them all off.

As for the kitchen cupboards, I've seen suggestions of filling under them with rock-wool type insulation. I'm not sure where the draught is originating from, it could be a gap where the pipes emerge or possibly it is coming down the walls. The room is insulated (it's an extension and it's the only properly insulated bit of the house) but I can see under the cupboards that the plasterboard doesn't come all the way to the ground...

The origin of the leak at the floor to wall junction is immaterial, be it from unsealed plasterboard, through unsealed block work, through unsealed stud work in timberframe, through the junction gap around turned up radon / damp proof membranes matters not a whit, it is the stopping of this air from entering / leaving the room that is relevant here.

Using air tightness tape will solve this issue for good and if done properly there should be no sign of the work when the skirting board is put back in place. All other methods such as foams, sealants etc, while okay maybe in the short term, will need to be revisited regularly plus also bear in mind that not only the bottom of the skirting board but also the top needs to be sealed if not using the tape method.

What's the construction of the floor?

Not sure exactly what you are asking here but if it is where to start to tackle heat losses in a meaningful way you won't go wrong with a good heat loss survey which includes a pressure test of the house.

Yes sockets, switches will all leak air (any penetration really of your walls, ceilings are likely leakage points, even internal ones depending)

No, other than sealing recessed lighting fixtures if present to the ceiling in the attic there is nothing that can be done.

General rule of thumb 1; you deal with air leakage from inside the space / room (the attic is outside the thermal envelope)

General rule of thumb 2; the air tight barrier layer should always be on the warm side of the insulation layer

spurred on by how freezing my kitchen was today I took off the kickboards and went looking for the source of the draught. I reckon this outlet from the sink/dishwasher is the culprit - it just disappears into the exterior wall and there appears to be a bit of a void below it as well. I used my infra-red thermometer on it and it read 11C, the rest of the room is around 20C. The further I go from the hole the higher the temperature is but there's a cold draught blowing out from under the cupboards and skirting all the way around the room and the floor is cold as a result.

I checked where the pipe comes out outside and it is sealed up but I assume the hole in the wall goes outside the insulated layer so air is getting into it anyway. Short of ripping out the kitchen cupboards to seal it up, is there any other solution?

Pack around the pipe (it's pretty inaccessible) or under the cupboards?

Anything to be said for expanding foam around the pipe if I could reach it?

Expanding foam is a filler, not a sealer.

@10-10-20's comment on packing the fibre into the gaps is the one to try. Also, don't forget the other floor to wall gaps under there no matter how small or insignificant looking.

Foam is a filler alright, but it will "seal" a large gap enough to prevent wind, it wont be airtight, but it doesnt need to be to stop a draught.

On the original question regarding skirting, a quick and easy approach is to just caulk the boards top and bottom, this just prevents the breeze for entering the room, but its still going somewhere (probably an open cavity wall in the attic) but its a lot simpler than getting into skirting removal etc.

Tho whole point of doing this is air tightness so using a material that inherently will not give an airtight solution (especially after it shrinks over time) is not the best approach I would have thought. Im my experience, using well compacted fibre will give a far better solution especially over the medium to long term and is safe in the sense of no chance of adverse chemical reactions between the foam and cable housing if cabling present.

Using caulk is perfectly fine for one heating season (i.e. very short term) due to the seasonal expansion / contraction of the different elements but will need to be addressed at least bi-annually, if not annually depending. It can also look a tad messy depending on the finishes. I know that if I was doing this I would want to do it once and not have to worry about it again!

I'd argue that the point of doing it is to avoid draughty walls. You don't need to be air tight to avoid a draught.

The "best" approach very much depends on ones appetite and aptitude. If a good quality caulk + gun and an hour gives the OP 95% of what they want then that is the best solution for them, especially if removing all skirting with the associated making-do will only gets them 5% more.