Agaricus species, most likely Agaricus campestris.
Note the fibrous cap, light pink young gills and large, pendulous ring.
Cap appearance is not entirely consistent with A.campestris, nor is the short, stout stem and obvious ring.
No yellow colour change on bruising, as would be expected from A.arvensis or A.xanthodermus. Some pink bruising around insect boreholes.
Found growing under hornbeam (Carpinus betulus) near grass.