Looking through a guide (Collins Nature Guide, Mushrooms and Toadstools of Britain and Europe) all the all the agarics I can find have free (detached from the stem) gills so your example is unlikely to be
Agaricus as the gills in your photo are slightly decurrent (attached to and descending the stem).
I can only find one example in the guide of a mushroom with decurrent gills and a ring on the stem, which is
Catathelasma imperialis
A quick google image safari turns up this link on RogersMushrooms.com:
http://www.rogersmushrooms.com/gallery/DisplayBlock~bid~5723~source~gallerychooserresult.asp
Your pics look a lot like an old version of this:
DISCLAIMER:I am not an expert - all I have done is look through some guides and do a quick google scholarship.
Rule 1: If I say something is poisonous assume I'm right.
Rule 2: If I say something is good to eat, assume I don't know what I'm talking about and that it's actually deadly poisonous.
Following these two rules will not cause you to be poisoned by my inaccurate advice.
I mean it about not being an expert so the above is purely for interests sake - always seek the advice of a real expert when it comes to fungi.