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.
We have a large Turkish/Iranian/Mediterranean hypermarket up the road from us. (London borough) They have a large comprehensive butchery selling all types of meat and offal including sheeps heads.
Possibly the used head was discarded and picked up bykids who threw it in you garden as a joke.
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