Neat though it may look I can think of things wrong with that so far (in the pics)
Space between shed bottom and base not high enough (think ventilation...think vermin!
) 4" for 4ft wide shed, 6" for 8ft wide shed is my recommendation...Based on years of designing, manufacturing and erecting sheds.
Shed looks rubbish to me but I would say that because....amongst other things all the sheds I manufactured had a good quality breather membrane between the cladding and the studs....Trouble is good quality costs.
Anyways you have what you have.
Vapour barrier......Looks like a dew point water trap to me unless your B&Q fibre wood underlay is vapour proof?....Wrong stuff anyways in my opinion, staples through to the outer cladding is like cavity wall ties without drip stops....Would rip it all out including the vapour barrier and fill in between the studs flush with proper insulation then vapour barrier over properly then final finishing board/surface.
If lining the ceiling then make sure everything is lightweight otherwise the roof might be liable to sag and the walls might want to spread out....
Roofing felt test...If you can easily rip the roofing felt with your fingers then its not really fit for purpose in my opinion.
Sorry to be blunt but I've seen it all before, was in the sawmilling/wood/shed/fencing game, many replacements/call outs for other folks's shed and DIY horrors.
Vapour barrier on the warm side of insulation...Not the cold side.
If you want to beef up and isulate the floor then floating floor is the easiest....Jablite, vapour barrier, floor panels...moisture proof T&G chipboard is what I'd go for, glue the joints...easy, "lino" or whatever on top if required.
To support floor joists, reckon on a max span between support centers of 1ft per inch depth of joist, you won't go far wrong with that.
With sheds also think storms....!
Good luck whicheverways.
Lawrence.