The police dont ask you what an unknown substance is at the point they arrest you - why bother? Its like asking a drunk driver how many pints he's had. They send it to be tested. It was dried mushroom in capsules. They just guessed £18 because they had never seen dried mushroom being sold before. They spent 6 months sending it to a lab to test and find out what it was - so clearly theres absolutely no shortage of money. Why not take it to court? Certainly looks better for them than a formal caution does.