And yet more than 90 percent of the capital's police officers carry out their daily duties without a gun. Most rely on other tools to keep their city safe: canisters of mace, handcuffs, batons and occasionally stun-guns.
This is no accident.
The Metropolitan Police, which covers most of London, was founded in 1829 on the principle of "policing by consent" rather than by force.
Giving everyday police officers guns sends the wrong message to communities, so this thinking goes, and can actually cause more problems than it solves.
Although there are higher numbers of armed police guarding Parliament, the attacker who rushed its gates in March was shot dead by a relatively rare member of the country's security forces — one who had been trained to use a firearm.
Some of these gun-wielding officers patrol the city in pairs, others are members of crack response teams — units dressed in body-armor, helmets and carrying long rifles — who are called to the scene of violent incidents like these.
In most instances, they don't use their weapons.
In the year up to March 2016, police in England and Wales only fired seven bullets. (Although these government figures do not include accidental shots, shooting out tires, or killing dangerous or injured animals.)
These officers fatally shot just five people during that period, according to British charity Inquest, which helps families after police-related deaths.
In August, when a teenager suffering an episode of paranoid schizophrenia killed an American tourist in a busy London street, armed police rushed to the scene but not a single bullet was fired.
They were able to subdue the attacker, Zakaria Bulhan, using a stun-gun. And no one else, bar 64-year-old American Darlene Horton, who had already been stabbed to death, was hurt.
The Metropolitan Police carried out some 3,300 deployments involving firearms in 2016. They didn't fire a single shot at a suspect.
It's a world away from the United States, where cops killed 1,092 people in 2016, according to figures compiled by The Guardian.
Of course it's easier for police to remain unarmed if civilians do the same. Out of every 100 people in Britain, fewer than four of them owns a firearm, according to GunPolicy.org, a project run by Australia's University of Sydney. In the U.S. there is more than one gun per person.
While British officials have long since accepted that an attack is "highly likely," they believe that intelligence-gathering and stronger links with the community — rather than gun-toting cops — will do more to keep the city safer.
"In a free and democratic society, there is going to be a balance between democracy, freedom and openness, and a police state — and none of us want to live in a police state," said Brian Dillon, former head of the Met's firearms command who now runs the counterterrorism consultancy Rubicon Resilience.
"Therefore at some point some attacks are regrettably going to hit home, that's inevitable," he added. "Not everything can be stopped."
While shootings involving police are relatively common in the U.S., authorities in Britain say they review each one with painstaking diligence.
Every time a British police officer shoots and injures or kills someone, it is automatically referred to an separate watchdog called the Independent Police Complaints Commission, or IPCC.
Although the officer who shot Wednesday's assailant has been branded a hero, they too will likely be referred to the IPCC, although that doesn't mean there will be a full investigation.
The watchdog told NBC News that it was "in contact with the Metropolitan Police" over the incident.
This process is not without its critics.
Some police have complained that officers are reluctant to sign up for firearms training because they fear being dragged through years of lengthy investigations in the unlikely event they have to use their weapon.
"Officers have seen what happens to their colleagues who have had to use lethal force to protect the public," outgoing Metropolitan Police Commissioner Bernard Hogan-Howe told reporters last month. "Increasingly, they seem to be portrayed as suspects, based, I can only assume, on an underlying belief that they must have acted in a criminal fashion if someone has died."
Families of people shot by police have also alleged the IPCC treated them insensitively, and in 2014 the watchdog released an internal review, "apologizing for our mistakes, and doing our best to put them right."
British police are attempting to recruit more firearms officers but the overwhelming majority will remain unarmed. Officials believe they have the balance about right.
"An attacker attempted to break into Parliament and was shot dead within 20 yards of the gate," Prime Minister Theresa May told lawmakers after the March attack. "If his intention was to gain access to this building we should be clear that he did not succeed. The police heroically did their job."