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).
The Royal Standard flag never flies at half-mast since the monarch is never dead. Whenever one king or queen dies, his or her successor immediately becomes the new monarch. "The king is dead, long live the king!"