The argument goes, as a British citizen, you have and continue to benefit from policies that your government made a long time ago. Reparations are not a tax on you, but an expense the government should have paid at the time of the work, but instead it did things like kidnap people from their homes, transport them to where labor was necessary, and force them into work. Now, the people who are the descendents of the kidnapped folks are requesting that the bills their great great grandparents were never paid. To extend that, after slavery ended, many of those who had been enslaved were left disenfranchised, and impoverished to the point that there is almost no possibility of building generational wealth.

As for if this will open the floodgates or not, who knows. An argument could be made in both directions, it’s not as though governments paying one time sums to places is rare, and reparations for wars used to be pretty run of the mill.