So in short, I don't think specie reserves show anything about Britain's ability to continue raising funds and continuing to support (financially, at least) the war.
This data is interesting. On the other hand, there's some evidence that the British government thought they were at their limit.
Ina memorandum written by the Chancellor of the Exchequer in 1809, who doubted that the war could be continued on its present scale. He concluded it would be impossible to retain Walcheren, send an army to the Continent, and pay further subsidies. "The difficulty therefore of supporting any considerable increase in the Foreign Expenditure of the Country, conspires . . . to establish the necessity of limiting the Scale of Operations, and of endeavoring as far as possible to confine the War to a War of defense."
By 18712, Wellington's troops had not been paid for five months, and his muleteers for thirteen. Castlereagh noted that "the scarcity of specie become the subject of much anxiety," and by 1810 Lord Liverpool wrote that, "We cannot expect to carry on the war on a large scale, without some difficulties, those of a pecuniary nature perhaps more trying than any other, but they are at the same time most common."
( I don't get that either).
In 1810 the British PM William Huskisson complained that "the demands on the military chest in the Peninsula . . . were such as to create the greatest apprehension that the chest would be entirely exhausted if the expenditure should be very considerably increased." For similar reasons he thought that the financial problems imposed by landing troops in Westphalia in 1809 would be insurmountable.
side note: Interestingly, in 1811, the First Lord of the Admiralty ruled out another attack on Flushing on the grounds that "The Peninsula and
Ireland absorb all we have and would do so were it double what what it is."
I wasn't aware that Ireland was garrisoned so heavily to keep it from rising up; and it seems weird to list it next to the Penninsula as a drain on troops.