According to this link to the British Museum website, the copy they have of The Ancient of Days is a drawing, not an etching with watercolour. They also state that it is not considered to be a work by Willaim Blake. http://www.britishmuseum.org/system_pages/beta_collection_introduction/beta_collection_object_details.aspx?objectId=6442&partId=1&searchText=the ancient of days
The Whitworth Art Gallery in Manchester, UK, do have the etching/watercolour as part of their collection. http://www.whitworth.manchester.ac.uk/collection/advsearch/objdisp/?irn=88&QueryPage=%2Fcollection%2Fadvsearchhttp://www.britishmuseum.org/system_pages/beta_collection_introduction/beta_collection_object_details.aspx?objectId=6442&partId=1&searchText=the ancient of days%2Fresults%2F&QueryName=BasicQuery&QueryTerms=william+blake&StartAt=1&QueryOption=all&all=SummaryData%7CAdmWebMetadata&Submit=Search
