Blink and you'll miss it. The tiny obelisk on the road out of the Hertfordshire village of Wadesmill hardly presents itself to the world, yet it is one of the most important monuments in British history. It marks the spot where the slavery abolitionist Thomas Clarkson had an epiphany: on that day in June 1785, on a journey from Cambridge to London, he committed his life to ending the transatlantic slave trade.

That marker serves as an appropriate symbol for the hidden history of the slave trade. The exchange in Africa of British goods for 3.25 million people (the international figure is nearer to 20 million Africans) to work the plantations of the West Indies and America is the single most important contributory factor in the country's accumulation of wealth in the 19th century. In one sense that history is hidden; in another, it is clear for all to see.

It is there in virtually every major building: the factories, mills and warehouses of the Industrial Revolution, stately homes, churches, banks -- the Bank of England, HSBC and Barclays have their roots in slavery -- the National Gallery, Guy's Hospital, the Houses of Parliament, the London docks, and Bristol and Liverpool. And it is in what we consume: cigarettes, coffee and sugar.

This trip round the Britain of the anti-slavery movement cannot be just a geographical one: it has to be a tour of understanding...