Anglo Saxon Chronicle for 871 records Alfred going to Reading and defeating the Danes at Ashdown and Basing (Basingstoke) Nothing is said about a white horse though. Hengist does mean horse and there is probably confusion with his brother Horsa - White Horsa anyone? Hengist had no connection whatsoever with Uffington being far to early. Uffington is Saxon and probably means Wuffa's to(w)n.

Geoffrey Ashe writes at some length about hill figures and Uffington in particular in his "Mythology of the British Isles". In his view, the white horse is of great age and he too draws comparisons with designs on Iron Age coins and earlier Bronze Age engravings. see http://www.themodernantiquarian.com/forum/?thread=13971&message=143516