Need help finding the right soil (uk)

I’ve been using the bugznbits substrate stuff but it seems to get very pricey. I know you can just do the whole organic topsoil no added fertilisers etc but I don’t even know what soil to get then?? How do I check if it has things added? Are there any (UK only) brands that are recommended? I have some soil it’s organic peat free topsoil. Happy to bake it too. But I’m mega paranoid and worried that it contains something that might be harmful :( please lmk 🙏