Been following this thread with interest.
I'm based at home and have been looking at Garden Offices from the normal places of approx 3mx2m/3mx3m and they were all coming in around £7-8k. After seeing your build, I managed to get a quote for something similar at Keops (either a Modena or Quanta) for around £3k.
Favouring the Modena at the mo as it's under the 2500mm limit so no planning permission costs/rigmarole.
My main concern with all of this is the temperature within the cabin. So, will it be too hot in summer/too cold in winter. Note that I would opt for roof/floor insulation with double glazing.
If I did find it too hot/cold, then it's seemingly a difficult task (due to wood swell), to create a void using a stud wall and insulate it. It also seems very expensive to opt for the PVC option.
On the flip side, I realise that at 6/9m^2, even if I did lose heat from the walls, it's not going to take a lot to bring it up to temperature and at a £4k saving over competitors... it's a lot of heating.
Decisions.