Does the rest of the stuff have any structural purpose or can it be cleared out relatively easily?
Yes, it's probably holding the rest of the house up. It can't really be cleared out relatively easily, it can be removed and the basement converted to a full one but it's expensive.
Our flat is a ground floor one in an 1865 terrace and has exactly this, but it's more earth than rubble. Our coal chute is still there as is the cover at pavement level.
I once heard a brushing noise outside and went to the door to find an elderly couple brushing the coal hole cover with a wire brush so they could take a photo of it. Apparently ours is a good one. They live up north and come down to London on the train to 'spot' coal hole covers.
Nowt as queer as folk.