The first steps of the tour are just before London Bridge. (This caused me a problem as I had my baby with me in her buggy.) The tour asks you to stop outside a church before London Bridge. The church may not be obvious as it is set back on the right. Have a look at this photo of St. Magnus the Martyr Church
to help you recognize where to stop.
I liked the information about London Bridge. I learned that this bridge was built in the 1970s after the previous bridge was sold to Americans and resides in Arizona. Is it true the Americans thought they were buying Tower Bridge