Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
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Tuber bellisporum is a non British fungi so I think this is unlikely. Not sure what it might be though.
Possibly the tuberous root of a plant, rather than a fungi? Just a thought.
I'd like to suggest the fungus commonly known as Dead Man's Fingers (Xylaria polymorpha - found on tree stumps, irregular in shape and white inside....
You could well be onto something with X.polymopha mandyson.
I might have agreed with Dead Man's Fingers but this I found on open soil with no obvious wood around, so i'm not sure about that identification.
Lat/Lng: 51.1867, -0.3378
OS grid ref: TQ162443