Perhaps you're thinking of:
* "The London Eye": This large Ferris wheel, often called the "London Eye," is located on the South Bank of the Thames and offers stunning views of the city. It might be perceived as a "mountain" of sorts due to its height.
* "The Shard": Another towering structure in London, the Shard is a skyscraper offering panoramic views. It could also be perceived as a "mountain" due to its height and prominence on the London skyline.
Let me know if you had something else in mind.