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).
There is no place in the United Kingdom that is more than 113km (70 miles) from the sea. The most landlocked place in all of Britain? Coton in the Elms, an English village of around 900 souls in Derbyshire, just NE of Birmingham.