An EL&U question from 2010 asks Which is the correct spelling: "grey" or "gray"? The answers very sensibly point out the split between the UK and former British commonwealth nations preference for grey and the U.S. preference for gray.
But if you look up the phone listings for the surnames Gray and Grey in an arbitrarily selected UK city (I chose Sheffield, Yorkshire), you get many more for Gray than for Grey. For example, for Sheffield, I got 52 matches for Gray versus 6 for Grey at the BT The Phone Book site, as of September 1, 2014.
A quick trip to Samuel Johnson’s 1756 Dictionary of the English Language reveals this entry for gray:
GRAY, a. [ʒɲæʒ, Saxon; grau, Danish] 1. White with a mixture of black. Newton. 2. White or hoary with old age. Walton. 3. Dark like the opening or close of day. Camden.
And this entry for grey:
GREY, a. [gris, French] See GRAY.
Does it follow that most Gray surnames were adopted on the model of the Danish grau and/or during the period of dominance (assuming that there was one) of gray in British English, while the surname Grey reflects a preference for the French antecedent gris and/or a later adoption of or switching to the current UK preference for grey? If not, what is the explanation? And when (if ever) did British English abandon Samuel Johnson’s evident preference for gray as the standard spelling of the non-surname word?