In 2005, the sale and possession of fresh mushrooms containing Psilocybin became an offence - the fresh shrooms are now a Class A drug. Before the "clarification" of the law, only dried or otherwise prepared mushrooms were outlawed. This was in response to the "problem" of an increase in the number of shops selling these mushrooms: usually Psilocybe cubensis. Did anyone notice an increase in the number of hippies wandering around, giggling during 2004? Was there a rush of admissions to hospital with magic mushroom poisoning in that year? Were shroomed-up thugs raping grannies, and stealing their pension-books to get their next fix of fungus?
There was no reason for these mushrooms to be made illegal, except the Government wanted to be seen to be "tough on drugs", and to send a message.
Naturally, there are a number of exceptions. Psilocybe semilancea grows on most sports pitches and sheep 'fertilised' farmland, and is extremely common in the UK. Possession is legal if it is merely growing in your garden. It is also legal if you can "prove" your ignorance and can argue that you were looking for edible mushrooms. As a result of these exceptions, there has never been a successful prosecution for possession of this mushroom: Frankly the police have better things to do than arrest people for a crime that has almost no chance of reaching a positive result and is as close to victimless as it is possible to get.
The other main effect is that it is now impossible to buy Psilocybe genus mushrooms, but Amanita species remain legal, the most popular of these is Amanita muscaria. These are mildly psychotropic but are also mildly poisonous, and there have been several admissions to hospital, though no deaths... yet. People are still trying to get high, but are doing so with more poisonous species. Whilst the Fly Agaric pictured above is unlikely to kill you, it does have some much, much more dangerous cousins, with which it shares a number of characteristics, especially in early stages of fruiting. It is only a matter of time before some young psychonaut mistakes an immature A. muscaria for A. phalloides a species also appropriately known as the death cap.
Nice one, Government.