What a fine specimen! We thought it was a Deer Shield (Pluteus cervinus), a relatively common mushroom that usually grows on rotting deciduous wood. It’s a match for the picture in the Collins Complete Guide that we use but other sources show a mushroom with a lighter colour so we’re not sure – it apparently gets its common name from its dull brown colour. All very confusing, and DNA studies apparently suggest that not everything labelled as Pluteus cervinus really is. We should have looked harder at the gills (which should have been free) and and the spores (salmon pink).
However, sporesmouldsandfungi suggested via a comment that it might actually be a Tricholoma, perhaps Tricholoma terreum (the Grey Knight). I though that was a fan of pine woods rather than beech but maybe ours was on a long gone pine stump. We’ll have to hunt around and try and find another one for detailed study.