As the vast majority of domestic roofs in the UK are timber then tiling the roof is not an issue.
If it were me. I’d use C16 ‘2×4’s for the stud work and rafters. I’d include a collar contecting opposite rafters at their mid point. I’d then clad the inner face of the walls with 18mm wbp ply and the outer face of the roof with 12mm wbp ply. I would screw it together to.
As for footings, I’d lay a reinforced raft raised 100mm out of the ground. I’d dig a 400mm deep, 400mm wide trench round the outside and then take 150mm deep out of the middle. whack down 100mm of MOT in the middle bit. Lay some reinforced mesh (held 50mm off the bottom) across the whole (with a 50mm gap around the edge). Then pour a slab.
Good solid base that’ll move as one if needed and provide a options for the internal floor finish.
Admittedly the most expensive route but it won’t distort or crack – a concern I would have on a ‘shed’ of this size.