A fabulous, inexpensive lunch spot atop tombstones in the crypt of St. Martin-in-the-Fields church on Trafalgar Square
Do not buy point-to-point tickets. What you want is an Oyster Card (possibly with a Travelcard transit pass).
It is considered to be an act of treason to place a postage stamp featuring the queen’s head upside down on an envelope. Off with your head!