There's a lot going on with tea in Harrow the Ninth.
It's "overwhelming" and "too much" for Harrow, yet John regularly summons her to sit alone with him and drink it.
You did not understand why anyone ate these biscuits or drank this tea.
In the scene where John gets Harrow to admit what her parents did to create her - nobody has to know! - the entire exchange is framed around descriptions of how much John is enjoying drinking his tea and eating his biscuits, and descriptions of how much Harrow does not want to consume them at all and yet feels unable to do anything else. She understands herself as "required to drink it."
When John tells Harrow about the Tomb and the Body, we again get multiple descriptions of his enjoyment of drinking tea. Harrow is having a much less enjoyable experience: "you had not known you were shaking until God himself reached out to still your wrist, so that you mightn’t spill your tea over your knees." He asks her if she likes poetry or biscuits, and she makes it clear she isn't interested in either. He insists she eats two biscuits, and begins to recite the Poe poem associated with Humbert Humbert's first love to her by way of reminiscing about who he buried in the Tomb.
In moments where Harrow tries to assert her own agency, tea is there too. When she tells John about the Saint of Duty and Cytherea's body, her tea is "stubbornly undrunk" and John's biscuit crumbles into his tea. John is drinking tea when Gideon finds him interrogating Wake, and when the game is up and Mercymorn and Augustine turn on him, they both smoke and tap the ash from the cigarette out into John's empty mug.
Conversely, we see John drinking coffee by himself in the Mithraeum kitchen when he's not interacting with anyone. Harrow is also offered coffee by Abigail Pent, and "accepted a cup, mainly to warm her hands." Despite Abigail being another powerful figure of whom Harrow feels wary, there's no sense of compulsion or discomfort in this offered drink (despite it otherwise being a situation of gentle compulsion). Harrow feels able to accept it on her own terms. Which brings it roughly in line with how Harrow feels about physical touch from both John and Abigail as well.
But Katakaluptastrophy, you might be saying, sometimes the tea is just tea! Yes, but sometimes the author was a secondary school teacher in the UK, where this is a popular video for explaining the concept of consent to teenagers:
