London is too vast to cover properly in a single day, so this itinerary deliberately skips the museums, markets, and neighborhoods that deserve a slower visit and focuses only on the icons you'd regret missing. It threads together the royal, political, and historic core of the city in one walkable-plus-Tube loop.
Start early at the Tower of London to see the Crown Jewels before the crowds build, then walk across Tower Bridge for the classic river view. Take the Tube or a riverside walk toward Westminster, stopping to admire Big Ben and the Houses of Parliament and stepping inside Westminster Abbey if time allows. From there it's a short walk to Buckingham Palace to catch the Changing of the Guard around midday. In the afternoon, head back east along the Thames to ride the London Eye for a panoramic view of the skyline, then cross the river to explore Borough Market for a quick lunch or snack. Finish the day at St Paul's Cathedral or the Tower Bridge area as the sun sets, with dinner and drinks somewhere along the Thames to cap off a full day of sightseeing.