Destroying angel mushrooms – as nasty as the name implies. A fungus which had a central role in a 2001 episode of Midsomer Murders, which included a reasonably accurately portrayal of the symptoms of destroying angel poisoning.
Not a fungus I have often seen around the west midlands, it may have been brought in with the wood mulch.
CORRECTION: identified by Lukas Large as rose-gilled grisettes