https://www.rothschildarchive.org/business/n_m_rothschild_and_sons_london/
excerpt:
Early business
"Temporary access to funds, invested by the Rothschild House in Frankfurt for William IX of Hesse-Cassel, greatly increased the scope of Nathan's London operations. These were based upon profitable speculation in British and foreign securities, and prominent dealing in foreign exchange and bullion. By 1814, Nathan was placed uniquely to fulfil the prestigious British Government contract to purchase and transport gold coin to finance Wellington's army on the Continent."
"After the latter’s victory at Waterloo, the London House won a further contract to handle English subsidy payments to the European allies. The position of Nathan Rothschild as the leading City merchant banker was consolidated in 1826, when the firm stepped in, with an instant injection of gold, to save the Bank of England."