The distinction between class liability and racial liability is fundamental to the reparations debate and almost never acknowledged.
If you wanted to construct a targeted reparations mechanism that matched actual historical responsibility, you would focus on the descendants of the specific class of people who owned plantations, invested in slave trading, and profited from enslaved labour. Some of those families can be identified. Some may indeed hold inherited wealth traceable to slavery. A targeted claim against those specific families would at least have logical coherence.
That is not what is being proposed. What is being proposed is a payment from Britain as a whole, funded by all British taxpayers, regardless of whether their ancestors had any connection to the slave trade. That proposal is based on racial identity rather than historical responsibility. And racial identity is not a legitimate basis for financial liability.