The Wellington Arch is near Hyde Park and is often something that people just walk under rather than visiting because they don't know that you can go up to the viewing gallery. It was originally it was designed to be where the Victoria Monument (fountain outside of Buckingham Palace).
It was built in 1826-1830, to commemorate Wellington's win over Napoleon. Now it hosts the biggest bronze statue on top of it in Europe and has a great view into the gardens of Buckingham Palace.
All the Best!
American to Britain