The garland roundhead (Stropharia coronilla) is a widespread species in the south of England and is quite common on lawns and grassy places. They can be solitary but, in general, they occur in small groups. They have a lovely shiny brown cap which often hides the stipe so it looks just like a brown lump on the grass. An autumn species like many fungi.
This species is known to contain harmful chemical substances and is best left alone.