More important than its cinematic merits, the movie Belle, which opens in the UK this week, contradicts the persistent belief that the black presence in Britain began with the Windrush generation in the 1950s. A lush and touching period piece set in 18th-century London, the film stars Gugu Mbatha-Raw as Dido Elizabeth Belle, a young mixed-race girl and the ward of the most prominent legal figure in Britain at the time, Lord Mansfield. She is believed to have influenced some of the most crucial legal judgments of the Georgian era. But in the 18th century black people were not an unusual sight; indeed it was estimated that there were more than 15,000 in London alone, a proportion almost equal to that of today.

The sugar and slave trade were the most lucrative industries in the empire and inevitably involved the movements of thousands of black people back and forth between Britain and its colonies. Thus the British navy was replete with black sailors – some of whom fought alongside Nelson as he vanquished Napoleon – and soldiers fought in the king's army. Black personalities were also evident on the stage and in paintings, as well as on the streets, where they worked as peddlers and prostitutes, chairmen and footpads.

Above all else, as Belle demonstrates, they were in the homes of the wealthy and privileged. Indeed, possessing a black page, who represented the wealth and exotica of the Indies, was the height of fashion. Hence the dilemma of the Duchess of Devonshire who, having been given such a gift, discovered that her husband didn't relish the idea of having his wife permanently accompanied by a black boy nearing adolescence. She wrote this desperate letter to her mother: "He is 11 years old and very honest … I cannot bear the poor wretch being ill-used; if you like him … I will send him, he will be a cheap servant and you will make a Christian of him and a good boy; if you don't like him they say Lady Rockingham wants one."

That many black people living in London were, as the duchess implies, "things", slaves who did not have full agency over their lives, is evident in the two pivotal cases presided over by Mansfield. The first of these was in 1772, of the slave James Somerset, who was rescued by the abolitionist Granville Sharp from his vengeful master, who had planned to sell him back into plantation slavery. It was a crucial issue because of how much money the enslaved population represented, both in Britain and in the colonies. Freeing the slaves resident in England would have meant relinquishing thousands of pounds' worth of property. So it was no wonder that planters and merchants became anxious when rumours circulated that the judge was enthralled by his black niece. When Mansfield eventually ruled in favour of Somerset, many felt their misgivings were justified.

Even though Mansfield's intention was specifically to make it unlawful for the slave to be transported abroad and back into chattel slavery, Somerset's supporters were jubilant as they interpreted it as meaning slavery was illegal on English soil. (The ambiguity of the slaves' status remained, however, explaining why Mansfield created a legal document that explicitly stated Dido was a free subject.)

Mansfield also oversaw the even more infamous Zong case in 1781, in which more than 130 living slaves were pitched off a ship on the Atlantic passage in order to collect the insurance money. The sheer venality and inhumanity of the slave traders so disgusted the public that it would eventually help to ignite the abolition movement.

If the history of Britain were made into a movie, black people would be in virtually every scene. From Roman times to the Tudor court, from the Georgian era to the present day, we remain a constant presence. It is time to stop airbrushing us out of British history and acknowledge our long and rich contribution to this country.