Five years ago, in 2015, the Apple Watch went on sale, water was discovered on Mars, and British taxpayers finally paid off their multi-billion-dollar debt to compensate slave-owners for lost income.

"The past is never dead. It's not even past," says tour guide Courtney Plank, beginning London's new "Slavery and the City" walking tour with a quote from American author William Faulkner.

Behind her, St Paul's Cathedral soars between modern high-rises, one of many London vistas where the 21st-century skyline is sliced open by monumental reminders of the city's antiquity.

Not all of Britain's living history is so visible, however.

The City of London, also known as "The City" or the "Square Mile," is both the capital's historic center and, for millennia, its business heart.

London has ascended to its spectacular position on the world stage by being the principal city in what was once the world's largest empire.

While, in an ordinary year, it attracts some 30 million tourists to enjoy its magnificent architecture and unrivaled cultural offerings, this glory has come at a price.

Summer of reckoning

Now, as part of a domino effect produced around the world by the death in May of George Floyd and the Black Lives Matters protests that followed, Britain's institutions have been reckoning with their own legacies.

The Bank of England and the Church of England are among many groups to have issued belated mea culpas for their historical links to the slave trade.

The British Museum, founded by physician, naturalist and slave-owner Hans Sloane, has announced plans to "contextualize" the more controversial items in its collection, often connected to imperial plunder.

The National Trust heritage body, which looks after many of Britain's beautiful and much visited stately homes, has also vowed to address the estates' colonial legacies and present visitors with less rose-tinted histories.

While most tourists looking for an educational stroll are pulled towards the pageantry of London's many Jack the Ripper walks and ghost tours, the Six in the City tour company -- based in the Square Mile -- takes a more erudite approach.

It was already working on a response to the new public interest in Black history when the opportunity came along to contribute to architecture festival Open House London, which was keen to diversify its 2020 program.

Now its new slavery tour is running during October's UK Black History Month and, if there is demand, well beyond.

Secret history

The two-hour walk sheds fresh light on popular tourist landmarks such as the Jamaica Wine House, a quaint redbrick building in one of the charming alleyways that pepper the financial district. The George & Vulture pub, as frequented by Charles Dickens, is across the way.

Built 1652, the Wine House is the city's first coffee house and was a favorite of diarist Samuel Pepys.

What the plaque on the outside doesn't tell you is that it's also where ship captains and plantation owners met to do business, buying and selling the lives of enslaved Africans thousands of miles away and arranging their forced transport to the New World.

Then there's the magnificent Portland stone pediment of the Royal Exchange, next door to the Bank of England, with its 19th-century tableau depicting imperial commerce (pictured at the top of this article).

Hidden on the right of the sculpture is an African figure on his knees. The slave trade had ended more than 30 years before it was carved, but colonial exploitation was far from over.

A modern glass edifice nearby houses the headquarters of the illustrious Rothschild banking family. It was the Rothschilds who in the 1830s arranged that recently paid-off £20 million loan to the British government to reimburse slaveowners after the trade ended -- said to be equivalent to some £300 billion in today's money.

Abolitionists fight for change

The tales shared are not just about the horrors of the slave trade, but also of the abolitionists, both Black and White, who campaigned for its end.

Guildhall, a magnificent municipal building with a history going back to Roman times, was the site of the Zong Slave Ship Trial following a massacre on board a ship in 1781.

Nigerian-born Olaudah Equiano, a freedman who had bought his way out of slavery, was part of the Sons of Africa group, thought to be Britain's first Black political organization. It was he who first brought wider attention to the trial, the publicity from which helped drive the movement for abolition.

Equiano's memoirs of his times as a slave and later as a sailor and traveler -- he's believed to have been the first Black person to have visited the Arctic -- were published nearly 70 years before the American Frederick Douglass' more famous slave narrative.

And in May 1787 at 2 George Yard, now a quiet anonymous courtyard, a printing house hosted the first meeting of the Society for Effecting the Abolition of the Slave Trade.

The house printed the famous diagram of the Brookes slave ship which, Plank tells CNN Travel, was a "well used and, at the time, pretty successful propaganda tool."

Silver pieces

St Mary Wallnoth Church, a stone's throw from the Royal Exchange, is where clergyman and slaver-turned-abolitionist John Newton, who penned the hymn "Amazing Grace," preached against the trade in the 1780s.

Powerfully, he compared the buying and selling of humans to the Bible lines about the silver pieces Judas collected for betraying Jesus: "It is not lawful to put it into the Treasury, because it is the price of blood."

The Jamaica Wine House also hosted gatherings of abolitionists, the most famous of whom is William Wilberforce.

"In the run-up to the final legislation on slavery there was a big uprising in Jamaica," says Plank. This also helped to galvanize public opinion in favor of abolition.

"There were so many uprisings on the ships," she adds, "which was extraordinary when you think of the conditions they were in and by then how malnourished and ill people were."

Plank is conscious that while it's important for Britain and other countries with imperial pasts to face up to and assimilate its colonial legacies, there is much more to the history of Black experience than the horrors of slavery.

She says: "We're going to try to do another walk which is more on Black history in the City, because there are other stories to tell."