It's worth checking out Paul Newman's Lost Gods of Albion for his take on Uffington.
If horse cults is your bag, there's a couple of interesting bits in Lethbridge's Gogmagog.
I was thinking about the whole horsey thing yesterday and was wondering if our common distaste of the eating of horse flesh that exists in these islands is the last remnant of the horse cults, although Lethbridge quotes an interesting account from Giraldus Cambrensis' Topography of Ireland which describes the inuageration of a king at Kenel Culin in Ulster (Tirconnell in Donegal). The ceremony involves the sacrifice and butchering of a white mare. The king is bathed in the broth prepared from the mare and fed the meat