The area around Bankside and Borough is blessed with a range of bollards but a common history. After defeating the French in the Napoleonic wars, the British had many French cannons. At about the same time (the first few decades of the 19th century), the local parish authorities in the Bankside area formed a works committee to improve the muddy, swampy lanes next to the Thames. The French cannons were recycled into humble bollards as part of the project. Their round top is an oversized cannonball!
Apparently, the shape was so popular cannon casts were used to make new bollards, one assumes, once the French cannons ran out. The bollards can often be seen with Clink printed on them, a reference to the Liberty of Clink, the closest thing they had to a local authority in the area, some with the date accompanying them.
Dr Cathy Ross, an Honorary Research Fellow at the Museum of London no-less, has written a history of the icon square mile, City of London Bollards. City bollards are not cannons but have a distinctive lemon squeezer head and have stars in their moulding, brightly painted red, black and white. However, if you know where to look, several City bollards have ended up South of the river, although they look distinctly more bashed up than their City cousins. Why they are here is disputed to this day.