E.g. here he's complaining to the undersecretary of state:
https://archive.org/details/dispatchesoffiel10welluoft/page/...
Here (and a few lines on the page before) is a long letter with his advice on how to reconstitute the (allied) government of Spain:
https://archive.org/details/dispatchesoffiel10welluoft/page/...
[1] I am sure this is not actually the right term. I do not care.
Don't know whether that's true or not (that the Duke of Wellington said that) but... One year later (1815), he handed the french's arses back to them big, big, big, times at Waterloo.
Basically the battle of Waterloo (a few kilometers away from where I was born) is considered the time when the UK overtook France as the world's number one superpower.
Since then both have only ever been falling in the rankings and it doesn't look like that fall is going to stop anytime soon but that's another topic.
But unlikely a result of said battle, rather the instability of politics in France.
Us British oft think of Waterloo as a great victory, although the circumstances, participants and objectives were pretty nuanced. Wellington himself rejected congratulations and thought battle to have very high cost.
That's an anachronism, from the 19th to mid-20th century there were just "great powers", not perfectly matched but considered to be in the same class. The Ottoman empire falling off the league ("sick old man") was a bit of a shocker.