if its a good priced shed the floor will be fairly strong
you could measure the "floor joists / bearers " and just add a few more
what are you going to put the shed on ? ? ?
as for insulation , well measure the upright timbers thickness, my guess they will be 2 and a bit inches, then go and choose the thickness polystyrene insulation ( it will come in 8 X 4 ft sheets, very easy to cut down to the right width ) then slid inbetween the upright studs, if you cut them fairly tight they will stay in place, failing that just buy some1/4 " or 3/8" ply and line the insides to hide the polystyrene
yes it will add a few quid onto the price but i bet the 19 x 10 isnt cheap, so it will be money well spent
this year i have project managed a "home office / guest suite build " built out of a nice big shed, we did exactly the same procedure but covered it in 3/8 plasterboard not plywood because it had to be a good finish, it was then chaulked and taped and painted, it looks like a house by the time it was done, had a bathroom and mini-kitchen aswell
it takes very little heat to make it nice and warm