For my whole time in London I have worked right down the street fro St Paul’s Cathedral. It is an amazing site and it truly sets up the skyline of London. You cannot take pictures inside so I just have a few of the outside here.
The cathedral was designed by Sir Christopher Wren in 1668. After the fire of 1666, the entire structure was demolished in the early 1670s to start again. It was the first building in London to have the dome structure. It was completed in 1708.
There is more information here on wikipedia. The site for visiting the cathedral is temporary and not fantastic. The cathdral has a very substantial crypt, holding over 200 memorials, and serves as both the Order of the British Empire Chapel and the Treasury.