One of the charming of St Paul's Cathedral is that you can see it from any alley near the landmark. How is it possible? First of all, because it is surprisingly big and marvelous.
St Paul's is a bic church that sits at the top of Ludgate hill, the highest point in the City of London. The church that you can visit nowadays dates from the 17th century: after the famous Great Fire of London in 1666 the Cathedral was involved in a big rebuilding program.
The cathedral is one of the most famous and most recognizable sights of London. Until 1962 it was the tallest building in London. Nowadays his dome continues being one of the highest in the world.
Due being in the City of London, it is funny to see between 12 to 1pm how office's workers go to have lunch in the small but quite cathedral yard in these sunny days.
Next to the cathedral, glued to her, you can go to visit the Millennium bridge. It is a steel suspending bridge that crosses the river Thames. In the film Harry Potter and the Half-Blood prince is showed how the Millennium bridge is destroyed. If you cross the bridge, on the opposite side, you can visit the Tate Modern museum, the most-visited art modern gallery around the world.
Back to the Cathedral, I suggest better you to visit the Museum of London, close the cathedral as well (just 300 meters from the church - in Barbican Centre-). Is a good way to know about the history of London and the UK in general. You will get acquainted with tools, clothes, traditions from the prehistoric times to day-to-day.
to get in: Tube red line or bus until St Pauls.
take care :)