-
Coal cellar would be most likely. Is there an external hatch to it?
The structural danger in doing more clearing depends on your foundation type. I'm guessing you have shallow brick/concrete footings? Is the existing cellar space already below the foundation footings? If so, tread carefully!
I imagine the rubble was simply a cost-effective way to build? The clear space above the rubble was deliberate in the same way you'd design a crawl space nowadays (easier to level, easier to access, etc).
I've been looking at a variety of houses recently and a few of them have basements. All are typical terraces dating back to about 1900-1910
The basements seem to be a corridor type clear area going from front to back equivalent to where the hallway above is and then the rest of the house footprint has about 3-4' of rubble or similar and then 1-2' of clear space below the ground floor.
How come they were built like this? A coal cellar and couldn't be bothered excavating the rest? Does the rest of the stuff have any structural purpose or can it be cleared out relatively easily?