Imo, it's always worth making the right mix for the job.
Am I correct in assuming that you want to do foundations around the outside, then have the middle be bare soil to plant into?
Rather than having a slab, and raised beds on top?
Around the outside, maybe dig 400mm down, put in 200mm of compacted MOT, and 200mm of C10 concrete mix.
And put some rebar in if you're feeling extra (it's not that expensive). Just remember that if you put rebar in, it is an absolute arseache to rip the concrete up afterwards.
Though tbh, unless you're really keen for the plants to root into the ground, I'd do a slab!