You don't need a ticket, an oyster card or an app to travel on the tube (and the bus, I assume). You just tap your bank card to enter and exit, and it automatically caps your daily spend. Magic!
Also if you have a long wait at King's Cross Station, the Costa in the Premier Inn just out of the door and slightly round the corner is a relatively calm oasis.
Also if you're beauty shopping, the little pharmacy at the back of Selfridges beauty floor has fabulous French and other pharmacy brands unavailable anywhere else.