We're in England, so naturally tea pops up from time to time. While tea isn't a major symbol by any stretch, whenever it appears, we're reminded that the story takes place in England, and also that Daisy—despite feeling so at home here—isn't actually from this country.
Tea works in one other way: It subtly tracks Daisy's sentiments toward England. At first, when she's thrilled to finally be away from her dad and terrible step-mother, Daisy's shocked and impressed that her cousins are constantly bringing her tea; it's a gesture of politeness that places them in stark contrast to those barbaric American children. Later on, though, in Part 2, Daisy makes the following observation:
Good old reliable English tea. Two world wars ago, battlefield nurses gave cups of tea to the wounded and it leaked through their bullet holes and killed them. (2.4.15)
Daisy's sarcastic commentary on tea at the end suggests a gentle disdain for the excessive politeness and civility of English culture that we haven't seen prior. She can't figure out what to say to Edmond, and the rosiness with which she once envisioned life with her cousins has taken a decidedly darker turn as she recognizes that the cousin she loves (oh so very much) is a shell of who he was before.