Getting on great with the cabin. Insulation may be needed, depending on what you're planning to do with it. If it's just for storage, then obviously not. But a home office or summer house then you'll probably need it. The cabin I ordered doesn't have too thick walls or double glazing. So will never be that thermally efficient. But as a workshop, that's fine.
I've not put the insulation in yet, I have some 50mm Celotex to go in as mentioned. I decided to put it on the inside, so I can do it at my leisure. Rather than delay the cabin build waiting for materials. I also wont go over the height of a permitted outbuilding, by adding insulation thickness to the roof. With building inspectors knocking about for other projects I've got going on, I didn't want to risk that.
£1500 seems so much. My brother in law put one up from Dunster House (I think) that came with pre-bonded insulation. But the Tuin ones do not. You'll get separate sheets of insulation, that you might as well buy from the builders merchant. It's just a case of sticking under or above the roof. Nothing hard to it. The floor will just be wooden joists, with insulation between and floorboards above. That will cost a little in timber. But again, no difficult to do. I watched plenty of videos on YouTube that take you through everything. The Tuin website has lots of guides too.