I was thinking of Wayland's Smithy,
Mithalwen, which, yes, is near the White Horse and Uffington.
Ancient Britain--Wayland's Smithy
Some of my summer reading about Tolkien mentioned a barrow he could have known in the area. I've forgotten the specific reference but assume this is it, but will tread back to the books to verify.
Is this one of those "had we but space and time enough" pond differences?