the other option is to dig 4 holes - one at each corner, and pour in concrete with a two 12mm threaded rods cast in to the top of each. Concrete to be about 1 to 2 inches proud of the surrounding land. Put 2 nuts and washers on each rod and level. Cut treated timber joists (if it's a six foot long shed then 6x2 will do) to the length of the shed plus an inch, and drill holes for bolts, bolt down joist, sinking nut into joist, and crop bolt.
Span between the joists at foot intervals with joist hangers and more joists of the same type and hey ho! you have a timber grid. 22mm wbp ply on top of that and then your shed goes down on top of the ply. The disadvantage of this is that the shed floor is raised, and the advantage is that it is dry. You can even insulate between joists by running sufficient strawberry netting across and down between the joists and loose-lay glass wool.....
If you need a ground anchor simply put a fifth concrete pad in cast in a chain which will come up through a small hole in the floor.