Well, that's not a very "strait forward" question. They have gotten it from several different places, some more than others, in different time periods. Which time period are you interested in? For a while, they imported (then exported) a lot from the middle east (turkey, Afghanistan, & others), Australia (Tasmania), Russia, France, and many other places. Their main target for export for a long time was China, and Chinese people began growing it themselves as well (against their governments wishes!). There have been a LOT of places Britain has imported opium (as well as other goods) from. Oh, and I forgot India! It was once a British colony and a large opium supplier!
Nowadays, I believe that Britain gets most of its opium from French and Tasmanian Pharmaceutical growers for medication production. They also import the Morphine, Codeine, and Thebaine already separated from the opium latex's "cocktail", to ease medication production. That way the raw opium need not be refined, and it saves a lot of time! Most of the illegal opium and heroin comes from Afghanistan and the Orient.
Just remember that Morphine, Codeine, Thebaine, and many others ALL come from the opium latex (resin). Some people think opium is one drug, in and of itself. It is NOT. Opium is like a "cocktail" or mixture of several different drugs know as opiate alkaloids. More drugs can then be refined and created from the original alkaloids. Such as:
Morphine can be refined into diacetylmorphine (aka Heroin).
Thebaine can be refined into oxycodone, hydrocodone, hydromorphone, oxymorphone, etc...
MANY other types of drugs too... That's just a few, but they all originate from (you guessed it!) OPIUM! ;)
they got it from India then manufactured the opium
The British wanted to trade opium for China's resources.
The opium war mainly began because of the British and China having an opium trade for tea. The British would get there tea if China got there opium. but the cause of the war was that china wouldn't accept opium anymore, and the british wern't okay with that.
The opium trade was extremely profitable for the British, and thus they wished to end the ban of opium in order to resume the opium trade and make profits.
If you're referrring to the opium wars between China and Britain, the British bought opium from growers in India (which was under British control at the time).
The British wanted to trade opium for China's resources.
The British won the first and second Opium War.
the British demand for tea.
They addicted the Chinese to opium, so the British can make a lot of money by selling opium to the Chinese
The Chinese wanted opium!
The British needed Chinese goods, primarily tea, but the Chinese didn't need anything from the British. That would leave a trade deficit on the British side, so they got the Chinese nation addicted to opium (unprocessed heroin) which they produced in India. When the Chinese emperor tried to ban opium, the Chinese people rebelled against him, backed by the British, which led to Opium wars.
Opium has been available for 5,000 years. Chinese indigenous opium was inferior to the opium grown by the British on Indian plantations, so opium users favored the imported product. The British essentially forced imported opium on the Chinese, and large numbers of people became addicted. The British used profits to fund its domination of India. The Chinese government tried to control drug abuse in the 19th century, but found it was impossible without British support. The British were very much against the Chinese growing their own opium. There is no reason to believe that poppies are not native to parts of China. See "Opium," by Martin Booth.