In 1793, the British felt gravely insulted by the Chinese reaction to the Macartney Embassy, but effectively accepted the insult.
In 1839, the British declared war on China for daring to clampdown on the Opium Trade. The resulting conflict was an extremely unequal curbstomp of China.
At what point between the two dates did the balance of power change between the two powers? When would be the earliest the British can win a war, even if it's a long, difficult, drawn out one? The British are free to use local leaders to plot against the Emperor.
Would the British holding onto the Philippines after the Seven Years War change the date?