If you live in an area of London with diverse cultural practices, the likelihood is that the skull is leftover from a meal or a ritual ceremony. Personally, I find it horrifying that this should happen.
Ringlet - It's likely that someone of Arabic or Mediterranean extraction had bought the head to use the brains and/or eyes for a recipe and thrown the skull out after.
It may, as advised, have been taken and abandoned by a fox, or by kids larking about.
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