An alternative that will work but takes some care is to use the standard houseplant potting soil as the beginning of your substrate. You want the cheapest, unfortified, no special growth additives potting soil, the stuff you would never get for your houseplants because you wouldn't want to need fertilizers after like you would with this stuff. If your prices are anything like here, you can get all you would need for maybe £1 or £2. It does need to be covered with sand or gravel to keep particles from floating around in the tank water.