A great walk in London is from Victoria station to Buckingham Palace, from BP to Trafalgar Square on the Mall (pronounced Mal) through the Admiralty Arch to Trafalgar Square, Nelson's Column and the National Portrait Gallery, then down Whitehall past 10 Downing St & Churchhill's underground bunker to the Houses of Parliment, the London Eye and Westminster Abbey. You could make a nice day of it if you stopped at all the museums along the way.
That is a great walk, You can also arrive at Charing Cross Station and then start at Trafalgar Square. Depends which train or tube line is most convenient.
The only problem is that Trafalgar Square is pretty much in the middle of the walk. If you start there, in order to see everything you'd have to walk between Westminster Abbey and Buckingham Palace and it's a fairly long walk with not a lot of interest on it, at least nothing as nice as a walk down the Mall with the parks on each side.
You could do it starting from Westminster station and do it in reverse, but Westminster is on the same Underground line (District) as Victoria, so it's six of one, half a dozen of the other.
