First, a word on British debt. One of Britain's biggest successes in the eighteenth and nineteenth centuries lay in the nation's ability to raise considerable amounts of government debt, compared to superficially larger and stronger sovereigns on the Continent. Part of the reason was the British ability to maintain a large tax base without massive political discontent. But what was also crucial was the success of British ministers in converting short-term floating debt, at relatively high rates, into bonds at lower interest, which were either redeemable in the long term, or perpetual. The South Sea Company, which led to the famous Bubble, was an earlier failed attempt at this strategy - but it was succeeded by the invention of Consols in the Henry Pelham ministry. Existing government debt was consolidated into Consols, which were perpetual bonds, originally paying 3.5% (later 3%). As inflation in these centuries was practically non-existent, these were an excellent investment for anyone who wanted a secure long-term income, and the various wars Britain fought over this period vastly increased their availability. This was a major step in creating the modern global public debt market. Until the mid-Victorian era, the vast bulk of global public debt was British.

(The National Debt: A Short History - Slater, M, 2018 - is good on this.)

In the particular case of the slavery-related loan made by the Rothschilds, the details of the loan are preserved in this debate in the House of Commons in 1835. (Joseph Hume, who instigated the debate, was a well known backbench Radical MP who often harried governments of all colours on any fiscal policy he considered irresponsible.) As can be seen from this debate, in return for the £15m of cash that the Rothschilds raised, the government gave them £11.25m of 3% Consols, £3.75m of Reduced 3% Consols (these were a class of bond which had originally been introduced to replace debt issued at a higher interest rate), plus, on top of that, a 25-year annuity, worth £101,875 annually, which represented the effective Rothschild commission for the transaction. The government's incentive for the transaction is partially alluded to in the debate. The government needed to raise a large amount of liquid cash in a short period of time, which could only be done by borrowing. If they had done so directly, the announcement of their intention in Parliament would have shifted Consol prices in the markets against them, affecting the actual yield they received. Relying on a single borrower allowed them to secure a better return.

In 1888, the classes of Consol were converted to a new kind of Consol which, after 1923, only paid 2.5%. In the twentieth century, inflation robbed Consols of their status as a good investment, and forced the government to rely more on floating debt - but there was no point in redeeming the old perpetual bonds, as this would mean replacing them with bonds with higher interest rates. But the super-low interest rates the UK experienced since the crash encouraged the Treasury to finally redeem the old perpetual securities, which they did over 2014-15. The securities which vanished then were not just the securities which paid for the slavery loan, but the securities which helped pay for almost every major British policy from the War of the Spanish Sucession to the First World War.