this is a load of utter bollocks.
the money was paid immediately in 1835, and the government borrowed to pay for it, as governments always do.
Government debt is in the form of bonds, which pay an annual dividend. This debt is bought and sold openly and changes hands many, many times.
Earlier government bonds were perpetual, i.e. they paid an annual dividend forever. This has nothing to do with slavery, it's just a form of debt.
A bond has a face value of £100, but sells at some discount or premium to that based on the bond interest rate, maturity, and prevailing rates. E.g., if prevailing interest rates are 10%, and you have a bond that matures next year paying only 1%, then an appropriate price of the bond is around £91, to make up for the fact that you get back £100 next year, and you'd earn £10 interest elsewhere, but only £1 from the government.
The government perpetual debt had rates typically around 3%, so if you bought a government perpetual then you'd get £3 each and every year forever. If you did so during periods when you could get 10%+ in the bank, then you'd not want to pay anywhere close to face value (£100), and the perpetuals traded as low as around £25.
Howeer by 2015, interest rates were rock bottom, and the government could borrow money over a very long horizon at rates below 3%, and as they had the right to buy back the perpetuals at face value (£100), they did so, rather than continue to pay 3% annual interest rates.
www.globalfinancialdata.com/gfdblog/?p=3306
They paid the slave owners in 1835, everything else is a red herring.