Basically, my understanding was a mixture of large particles will improve water flow, a collection of small, same size particles will impede water flow. So a fine sand won't help drainage, according to what we were told. Any barrier to water flow, such as large stones will impede water flower but in a container, the base is a large barrier.
With containers it's worth remembering that even a leaf can block the hole/holes making your container a boggy pond no matter what you put in on top. I wish that could sum it up but it really doesn't. I just wanted to share the RHS link there. The blackcurrant bit isn't really important but there are many people that would want to grow a blackcurrant in a pot if they don't have a garden to put on in.