For additional Sherlock-specific context, Mrs. Hudson keeps insisting to John and Sherlock that she is not their housekeeper, but often prepares food and tea for them anyway because she is fond of them. "Would you like a cup of tea?" implies that she is willing to bring Mycroft a cup of tea as well, which he readily accepts -- remember that Mycroft is a wealthy, powerful man, and is used to being served by people working for him. Her curt response -- "The kettle's over there" -- tells him that she is not willing to serve him, and invites him to serve himself if he wants. Apart from this being her way of reminding Mycroft that she's not a housekeeper, this is also her way of telling Mycroft that she is angry with him for withholding some information from Sherlock (this was revealed in the previous scene). So, she's showing her anger with him, but isn't being rude about it -- they both understand that she'll forgive him eventually, but not yet.