The claim is that enslaved people performed labour for which they were never paid. That unpaid labour created value. The recipients of that value, the slave owners and their successors, are still in possession of it or its proceeds. The people whose labour was stolen are owed compensation for it.
It sounds plausible because it describes a real transaction. Labour was performed. No payment was made. That is straightforwardly unjust.
The problem is translating this moral observation into a present-day financial claim against the British government or British taxpayers.
Labour performed by specific individuals was stolen from those individuals. The individuals concerned are dead. As discussed elsewhere on this site, the debt cannot automatically transfer to their descendants simply by virtue of descent. Descendants are not the same legal entity as the original victim, and they have lived their own lives in circumstances shaped by many factors other than what was done to their ancestors.
The payers have the same problem in reverse. The British taxpayers and citizens who would fund any reparations payment are not the same legal entity as the slave owners who extracted the unpaid labour. Most of them are descended from people who had no connection to the slave trade. Taxing them to settle a debt they did not incur is not restitution. It is the creation of a new injustice in the name of addressing an old one.
There is also the question of what has already changed. The 1833 Abolition Act freed enslaved people without compensation to them, which was wrong. But the British state has also spent considerable resources since then on aid, preferential trade arrangements, and development assistance to Caribbean and African nations. These are imperfect and inadequate responses to an enormous historical wrong, but they are not nothing. The claim that absolutely nothing has been paid or done is not accurate.
Finally, the labour stolen argument runs into a calculation problem that its proponents never resolve. How much labour was stolen? Over what period? At what wage rate? Using what currency? Discounted at what rate to the present? Every answer to these questions is contested, and the figures produced by different advocates vary by orders of magnitude, which tells you that the calculation is not economics. It is politics with numbers attached.
The one-line rebuttal: the labour was stolen from specific people who are dead, and their descendants are not the same legal entity as the original victims.
