Well, in order to conquer someone, you need two things:
The British have never, at any point in time, had both of those be true at the same time.
Wanting to: The British, be it the people or government, have never had a reason to conquer China. Trade was far too lucrative to lose it to attempting to change some colors on a map, and so they were content to take a few trading outposts (like Hong Kong and Shanghai) and force a few unequal treaties on the Chinese to ensure that business remained good. Occasionally troops would march into Beijing or similar areas for one reason or another, but in the end it was all to keep things flowing as usual.
Capability: Unlike, say, India, China has had a unified government and military since Qin Shi Huang Di founded the Qin Dynasty in 221 BCE. While there have been relatively brief periods of civil war, rebellion or other unrest, none have occurred during a time during which the British would have been in a position to conquer anything until the late 1800s with the Boxer Rebellion. During that time, however, the other great powers would have never allowed such a thing to occur, so any British who would have wanted to do so had to content themselves with control over the ever important Chinese trade.