Even on the foggiest of London days, it is almost impossible NOT to see the London Eye from dozens of locations within central London. This ferris wheel, also known as the Millennium Wheel, is one of the largest observation wheels in the world towering 443 feet (135m) above the Thames River. From the Eye (assuming a clear English day), you can get a fantastic 360 degree view of London. Many landmarks are easily recognizable such as Buckingham Palace, Parliament Building, and Big Ben. Westminster Abbey, The British Museum, the Tate Modern, and many others are also close by.
The interesting thing about the Eye is that it actually never stops moving, even when passengers are exiting or embarking. The Eye moves slow enough that one revolution takes about 30 minutes to complete. However, if there are slow passengers such as the elderly or disabled, then the Eye will stop to accommodate them.
The London Eye is very accessible and is within walking distance of many major sites. Even though it is not a highly adventurous ride, it does provide an excellent opportunity to get an overview of the London landscape.