How do you benefit from a fall in interest rates in general as a creditor of public bonds? I’m reading about England debt in 1819 and how interest rate benefited its creditors. The country had a public debt of more than 250% of the GDP.

With peace at last achieved, London is faced with a mountain of debt that it intends to repay.  And for good reason: it is owned by the aristocracy and the most influential members of Parliament (they will not fail to make huge gains on their &quot;consols&quot; (consolidated bonds) thanks to the fall in interest rates,  the Rothschild's fortune dates from these days)

How does the creditors of those consolidated bonds benefited from a fall in interest rates ? How do you benefit from a fall in interest rates in general as a creditor of public bonds?
This is because bond prices increase when interest rate falls.
A general bond pricing formula can be expressed as as a discounted stream of future cash flows from the coupon and the value of the bond itself. Hence we have:
$$ P = \sum^T_{t=1} \frac{C_t}{(1+i)^t}+\frac{M}{(1+i)^T}$$
Where $P$ is the price of the bond, $C$ is the coupon $i$ the interest rate, and $M$ the value at maturity (i.e. the face value). You can also learn more about the formula here if you are interested.
As you can see from the above formula when $i$ increases $P$ decreases and vice versa.  Hence fall in the interest rate is good for you if you are a bond holder as now you can sell your bond on secondary market for more than before.