Amanita citrina var. alba
Earlier this summer, Emily found (and posted) Amanita phalloides var. alba - The Deathcap.
Completely by coincidence the False Deathcap I found in the wood today is also the white form - as opposed to the more common pale yellow form var. citrina.
The books all say "associated with Beech", however this specimen was growing at the base of an Oak.
Like it's more deadly cousin it also contains alpha-amanitin, but not in a high enough concentration to be fatal (though I'm not about to put it to the test!).
Unlike A. phalloides it can easily be separated by the absence of an open volva.