>And the other way round: For sure, Opium was imported to England. Why didn't it become a problem?
Opium was produced around Calcutta(Kolkata). Shipping it to China was cheap, shipping it to the UK was expensive. Only the wealthy British could afford opium, and they used a ton of it. See https://en.wikipedia.org/wiki/Confessions_of_an_English_Opiu...
Well, in that historical case, the sale of Opium to China was a state sponsored project. Opium was only a minor problem in England, but it wasn't being smuggled into England by a quasi-hostile foreign country trying to re-balance its trade deficits.
The drug dealers were British subjects, in general. China didn't have an easy way to punish them (the actual merchants responsible generally wouldn't be entering China), but Britain absolutely did, so could enforce its own laws against opium (which were then fairly strict; it would've been a death penalty thing). Britain _did_ have an opium problem, incidentally, but the volumes imported were just much smaller.
Opium was produced in British India, not China, for the most part.
And the other way round: For sure, Opium was imported to England. Why didn't it become a problem?