There are many beautiful roads and places in London - particularly outside the usual tourist ways. But I cannot conceal that the contrast between old and new fascinates me specially at Westminster.
On one side the venerable Big Ben with its world-famous clock, on the other side the ferris wheel just on the opposite site of the River Thames. From up there you surely have a magnificent view over the city (if you like to pay the 12 pounds admission fee, of course).
And then there still is the Trafalgar Square to see with its pigeons and fountains, naturally the Buckingham Palace (not that it is particularly beautiful, but you just have to see it) with the adjacent St. James Park. Just take your time. And if there is any time left, try Oxford Street for shopping or Charing Cross Road for books. Not to forget Covent Garden. You might go to Notting Hill as well, there is a nice flea market. And... well, I guess you have to buy yourself a city guide. :-)