An artificial ceramic stone also called Lithodipyra (from the Greek for twice-fired stone). It was developed and manufactured by Eleanor Coade (1733-1821) in Lambeth, London. Coade stone is fine, hard and weather-resistant and was used for architectural decoration, memorials, statues and garden ornaments.
Local examples (click to enlarge)
Bungalow, Summer Hill, Harbledown – the two heads seem to depict Green men (q.v.)
Another possible Coade stone head on Summer Hill – awaiting confirmation