The gills of the oyster mushroom, Pleurotus ostreatus.
The upper surface looks like this:
This yielded the following spores (magnification x600, field width 86um):
These are about 8.2 x 3.3um which agrees well with the 8-10.5 x 3-3.5 µm given for P. ostreatus at MushroomExpert.com.
Specimen taken in Whiteknights Park, Reading, UK, on 2013-12-15.