Letter to Telegraph 13.9.13, from Philip Roe, St Albans:
“Allison Pearson’s ideas about how Shakespeare would write Downton Abbey were ingenious and amusing, but included a common misunderstanding: she has Carson address Lord Grantham as ‘thou’.
“Thou in Shakespeare is a bit like tu in modern French. Romeo and Juliet, for instance, address one another as ‘you’ at first. We think of thou as a way to address God because that is the form in the 1552 Book of Common Prayer. However, the decision to address God as a father, rather than as a Lord, was not taken lightly. It was a matter of intense controversy at the time.”
As I think I understand it, the plural and formal of Thou is Ye, which became You. Thee is for when Thou or Ye is or are the object of a verb. More explanation at http://en.wikipedia.org/wiki/Thou/