"...naval historian Martin H. Evans has comprehensively determined that it is spectacularly unlikely that any French weapons made it back from Trafalgar to become street furniture, it is true that, dating back to the 17th century, many iron cannons did undergo this transformation, often with a cannon ball jammed into the end to provide a nice round top. Captured foreign cannons were particularly good candidates because they took different-sized ammunition, and you can still find at least one (most likely) French cannon outside St Helen’s Church off Bishopsgate."
Old cannon re-used as bollards
Martin H. Evans.
http://people.ds.cam.ac.uk/mhe1000/bollards/cannonbollards.htm