Amanita citrina False Deathcap has a yellowish cap but this find is quite white. Kibby says that var alba seems restricted to Fagus but in Pembrey Country Park this was under Willow and Pine.
Kibby (the genus Amanita) describes what features to look for to help with ID. With citrina and var alba there is a prominent bulb at the base of the stipe. The top of this bulb forms a 'gutter-shape' as in the photo.
This is the first time I have found any Amanita species in the Country Park/forest.