According to the Miss Manners' Guide to Excruciatingly Correct Behavior" -- which I happen to read cover-to-cover after finishing Jane's Passenger Planes on my last non-stop rtw -- Judith Martin indicates that the phrase "She's rather milk-in-first, dear" refers to the fact that nannies and governesses seem to be prone to pouring the milk in before the tea, while all others do the opposite. In her description of serving tea, she says, "[A]sk your guest of honor how she takes it. Using the strainer, pour straight from the pot for strong tea, and dilute it from the kettle for weak. Then add sugar ... and milk, according to her taste."

In Aviation Etiquette Faux Pas and Other Misconceptions it clearly states that it is tea to be followed by milk. Sources in the British Royal Household confirm the information as correct with their comments: "We would not make any amendments to the text" and "What a good book with lots of big pictures."