These berries grow on a plant whose multi-pointed leaves resemble holly. But, as far as I can tell, the holly that grows in Europe (ilex aquifolium) has red berries much like its counterpart in the Americas. As you can see, these berries are blue. So I'm stumped.
This plant grows inside one particular section our bay hedge out by the back gate. The green leaves you see in the photo belong to the hedge, not the "holly." I should go take a photo of the pointy leaves, but I'm lazy. The fruit resembles blueberries, except that they're much smaller and don't have the blueberry's characteristic crown on the end. Besides, blueberries wouldn't grow in dry clay soil, I don't think.
In any event, I'm not eating them.
UPDATE: So I think the commenters are correct. This seems to be mahonia. And the berries are edible, once cooked. There aren't enough for me to bother with, so I won't. I suspect some bird ate the fruit of this ornamental plant somewhere in our area (apparently birds love these berries) and "deposited" the seeds here under our hedge.