There's no reasonable way to say this without sounding like a conspiracy theorist, but if you turn over a lot of bollards in London, UK, you will find that a surprising number of them are cannons.
Granted, any number of cannons above zero is a surprising number of cannons to find embedded in the sidewalk, but in the southeast and city of London you can find quite a few.
A reasonable question at this point might be "why"? In short, it was a way of recycling old defunct cannons. After a life of bombarding ships, the decommissioned cannons were shoved into the ground where they will be jumped over by drunk people and kids, unaware that they briefly had an actual 19th Century cannon pointed right at their genitals. The cannons, when buried, were either left open or sometimes stuffed with an oversized cannonball in the muzzle to seal it off.
There are those that claim the cannons were taken from the French at the Battle of Trafalgar in 1805. According to the tale, the British attempted to fit the French cannons to their ships but could not modernize them, and instead decided to use the cannons as street bollards as a way to taunt the French. You know there's nothing more offensive to a French person than revealing you've made pavement markers out of their weaponry.
However, this is unlikely to be true, as no captured warships were brought back to England. In fact, it was common practice to throw cannons overboard during a storm on "prize ships" in order to lighten the load. One such storm took place, requiring prize ships destined for Spain to lighten their load in this way.
Nevertheless, it's cool to know that there are bollards out there that are cannons (or sometimes replica cannons), even if they weren't placed there to taunt France.