Swallow is quite common in the west country - Swallowcliffe overlooking the Dorset Cursus, and the swallets on Priddy. Holes in the limestone that used to "gurgle" and make strange noises, there have been bronze age deposits found in some swallet holes. So swallowhead is a bit like winterbourne, it reflects the water coming and going - its descriptive. Thats my explanation anyway...