I have ordered a shed and whole slab pavement/patio seem like a big job I ordered plastic based 8*6 shed base with membrane. Now I want to lay it, some suggest we can just lay builder’s sand as base, level it and but membrane with shed base on top. I am based in UK and it’s always rainy as soil is moist? What can be best way to do the job before shed and people arrive to assemble it?
I don't know where you ordered your plastic shed base from, but guidance is given here https://www.plasticshedbase.co.uk/content/6-how-to-build-a-plastic-shed-base- on what to put beneath it. You're still going to need to dig out, compact (probably with a whacker if your soil is not clay). lay heavy duty geotextile membrane, backfill with aggregate or hard core of some sort, with a layer of sand on top. Check what instructions the company you ordered from gives on this subject.