Any quality bagged organic soil will work. I started out with a combination of fox farm ocean forest and sunshine mix #4. FFOF is a bit stout for seedlings or even freshly rooted clones so either use a generic potting mix or starter soil or just cut down the OF with coco and perlite. Always add in some extra perlite and worm castings if you can.
Ok well if you are in the UK I can see why it's so hard to find a decent organic soil mix. When you search for it on Amazon your soilless mix comes up as top choice. Biobizz all mix looks decent; I would go with that if it's all you can find:
https://www.amazon.co.uk/gp/aw/d/B0...ganic+soil&dpPl=1&dpID=41BkX415jNL&ref=plSrch
Maybe also get the light mix for starting seeds or clones.
After your first harvest you can just keep recycling it over and over from there. Aquire some amendments and compost for recycling your soil in the meantime. If your plants need a boost brew up some compost tea and/or give a liquid fish and/or seaweed fertilizer. Water only as needed the rest of the time. No ph-adjusting or or bullshit flushing is required.