The monument for the Declaration of Independence was only built by the US Government in 1976 and is a series of inscribed stones. It is close to the Washington monument, erected to commemorate George Washington.

That is a big obelisk, so if that's the one you are thinking of, I'd guess the English one you are after is Cleopatra's Needle in London? That, however, was erected to commemorate the British victory over Napoleon. There are a load of smaller obelisks all over the place but I'd be surprised if any of them were erected to commemorate the loss of the American colonies.

By the way, it is widely accepted that the US Declaration of Independence was inspired by and largely based on the Scottish Declaration of Arbroath.