Does anyone know anything about gardening?

All right, one materials list coming up. For best use of timber make it just under 8' square, 2.4m to be precise.

You will need:
4×2 regularised treated for joists and ends, joists spaced at 40cm centres. 5 4.8m lengths and one 2.4. Approx £65

10 4.8m 150mm deck boards (allows for cladding the sides). Approx £120

500 deck screws, Screwfix code 73014 £20.

100 structural screws, Screwfix code 6167x £21.

A bit of landscaping fabric to suppress weeds beneath. Approx £10.

5mm spacers to evenly space the boards and take out the inevitable bends in them. I use 5mm hardwood, remove them as you screw down each board.

The pad feet sound a good idea, presuming they will take a 4x4 post. You will need a post in each corner and a central one fixed to a joist. If I were building this I would put an extra post halfway down each side, but I think you'll be fine without as it's for light use. I always use stainless screws, but no need for this job.

Make up your 2.4m frame, get it square, mark the joist centres at 40cm, then simply cut the joists ACCURATELY to fit (any deviation of more than 1mm will bend the frame and the boards won't fit precisely). Screw the joists to the frame through their end grain. You should be able to slide the entire framework up and down on the posts in the pad feet to level, screw to the posts and saw off the excess.

Disclaimer: this is the cheapest method, I use all stainless screws and more posts concreted in. I also fit the frame to the posts by cutting an L shape in each post to precisely cradle the frame, which takes time.

Also, I know this is sneaky, but if there is a ban on poured concrete you could get away with dry mixed concrete rammed hard around each post. In which case, holes 2' deep, 1 25kg bag of ballast mixed with 1 shovel of cement per post, total cost £20.