As to the British Army, it was loaded with hardened veterans of many campaigns and they were some tough hombres.
Wellington's Peninsular army was dispersed in 1814 after Napoleon's first abdication. Some of it went to North America to fight the U.S., some of it went to British colonies like India where the garrisons had been on station without relief for years.
The British troops at Waterloo were of lesser quality than Wellington had led in Spain, some of them raw recruits.
Wellington did have some experienced troops in his army, but many of them, like the Dutch-Belgians, had been in Napoleon's army as recently as 1 1/2 years before. Would kind of make you nervous.
But the “sepoy general”, as Napoleon liked to deride him, held it together and sent Napoleon on his path to the South Atlantic paradise of St. Helena.
Well, Wellington always said that Waterloo was won by British Gin is a little tough. It is well to remember that the British Army was spread prety thin and that discipline was responsible for the victory not the native toughness of the troops.
At the end of the day Wellington played a weak hand of cards very well indeed.