British English has many phrases for rules that everyone knows but nobody has written down.
It’s not the done thing: It is not acceptable, although no rule forbids it.
Bad form: Behavior that breaks an unwritten rule of decency.
It goes without saying: It is part of the arrangement, although nobody said it.
Taken as read: Assumed to be agreed without being discussed.
A nod and a wink: An understanding that is never put into words.
To read between the lines: To find what was meant but not stated.