My advice
you mention you want to use it as an office?, I guess thats all year round ? ? ? all day ? ? ? and I guess them answer will dictate how much you want to throw at it
right, insulation will be your biggest friend here, when the shed is erected, you will have space between the uprights, fill that void, if the void is 40mm, fill it, you can get 40mm Polystyrene ( you can get 35mm and 25mm ) just fill that void, then staple your foil bubble wrap on the insides of the uprights, tape the joints, this acts as a moisture barrier ( keeps it out, will also keep the warm air in )
I have just searched and found the upright sizes ( its 28 x 28 framing ) so use 25mm Polystyrene, if you really want it warmer and dont mind spending a little more, you could buy some roofing batten ( 25 X 38mm ), plant it on the framing and use 50mm Polystyrene, 50mm and the foil bubble wrap and you will be lovely and toasty
yes 9.5mm Plasterboard for the walls, get the tapered edge board and then you can tape and fill it
Now the insulation on the roof, once again, as much as possible, fill that void and use bubble wrap, the boards on the roof is a tricky one, that roof will not be designed to carry too much weight, maybe look at the really lightweight T&G boards, B&Q are always the thinest, its cheap crap and will have knots and flaws, but it super lightweight and its what I fit on converted shed roofs, 9.5mm plasterboard is heavy
https://www.diy.com/departments/spru.../139314_BQ.prd
you can paint it a nice pastel colour and it looks nice imho
the trouble with putting the hardboard down over the YBS, it'll flatten it and the air bubble will squeeze out, thus little insulation, if height allows and it will be tight, run some 25mm roofing battens on the floor, fill the void with 25mm Polystyrene, foil bubble wrap over the top ( maybe even under the Polystyrene aswell ), then 12mm OSB over the top of it all, you won't need the hardboard, you can put the laminate / carpet / LVT flooring directly over the top of the OSB
you have then basically changed your shed to a cheap timber framed room, sure its not as insulated as a house, but its a hell of alot more insulated than a shed, spend the money now, get it right and it will keep you warm
of course, them windows are never going to be that warm, you might decide if you are spending 8 hours a day In it, see how it goes, if they lose alot of heat ( and they will ) change the acrylic sheets for double glazed units ( but you are talking a couple of hundred quid for that )
I would be happy to move into my mancave, safe in the knowledge that the 1500W heater works well and will keep the room warm, Infact my oldest has said when she Comes back from Uni, she could move in, it needs a toilet and shower, a small kitchen pod and it'll be good enough to live in ( with the 4 X 3M addition ) but it comes down to insulation and more insulation