Sacred Ibis Threskiornis aethiopicus
This distinctive species is common in captivity and escapes are occasionally encountered in the wild. Also a growing feral population exists in Europe, mainly in western France, and it's possible that birds from this population make it to the UK sometimes.
The first photo below shows a tame individual spent a few days near Dereham and was presumed to be an escape from captivity. The last set are of another presumed escapee in Norfolk but the rest are from Africa where the species is native.