I'm sure I've not seen this before although I walk past this tree almost daily. It is high on a veteran oak. Even resorting to digi-binning the final photo it is not easy to see what the topside looks like.
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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Lat/Lng: 51.418800368814, -0.95684051513672
OS grid ref: SU726693