I wonder what they're expecting? The British Empire was the first major world power to abolish slavery, decades or, sometimes, centuries before the others. They imposed a death penalty on slave traders, and the Carribbean island of Tortola (now in the Virgin Islands) was home to Kingstown, the first colony of freed slaves. The biggest opponent to the abolition of the slave trade was the royalty of Lagos, the ones who were actually farming the slaves and selling them; it took the British Royal Navy to put an end to them once and for all. The debts incurred by the British Empire from buying the slave's freedom - and the Royal Navy policing the Atlantic and Carribbean seas looking for slave ships - were only paid off late in the 20th century.
If they were serious, they'd be commemorating what the British Empire did to end a millenia-old tradition of people as property. Maybe they could do it in the crumbling ruins of St. Phillip's Church?