I’m not familiar with the stores in the UK, so I’m not sure which of those would offer great succulent soil, and my favorite pre-mixed soil is available only in the US. I can teach you how to make your own, though!
The most important thing for well-draining succulent soil is to have a large particle size – 1/4 inch (6 mm). The large particle size allows water to drain quickly and not sit on the roots for too long. Too much exposure to water can cause the roots to rot.
The mix I use for indoor succulents includes:
If you don’t have these available to you, you can substitute with coconut coir, pumice, or something similar that has a large particle size. My suggestions would be to mix about 50% organic material (some sort of bark, approx. 6mm in size) and 50% inorganic material (some sort of rock, also approx. 6mm in size). You should be able to find something at your local nursery.
Good luck! Let me know what works for you!