Parasol mushroom - macrolepiota procera - a relative of shaggy parasol macrolepiota rhacodes. ? that's my guess which I've just looked up in a book by the way. Still a lot to learn about mushrooms
apparently the Parasol mushroom has a smoother stalk and bruises red. The parasol mushroom is edible and good and can be between 4 and 10 inches across the cap which would fit the size of yours but avoid the shaggy parasol - that's what it says in the book anyway