A small white oysterling mushroom, probably Crepidotus cesatii.
I took a specimen. This is the upper surface:
And this is the lower surface:
It yielded these spores (magnification x600, field width 86um):
These are about 6 x 5um which agrees reasonably well with the 6.5-8 x 5-7um given for C. cesatii here but is too broad for the 5–7 x 3–3.5um given for the similar-looking Crepidotus variabilis at RogersMushrooms.
First photo and specimen taken in the Wilderness, Whiteknights Park, Reading, UK, on 2014-01-11.