After the British Raj took over in 1847, colonisers added a special new twist to the tax-and-buy system. As the East India Company's monopoly broke down, Indian producers were allowed to export their goods directly to other countries. But Britain made sure that the payments for those goods nonetheless ended up in London.
How did this work? Basically, anyone who wanted to buy goods from India would do so using special Council Bills - a unique paper currency issued only by the British Crown. And the only way to get those bills was to buy them from London with gold or silver. So traders would pay London in gold to get the bills, and then use the bills to pay Indian producers. When Indians cashed the bills in at the local colonial office, they were "paid" in rupees out of tax revenues - money that had just been collected from them. So, once again, they were not in fact paid at all; they were defrauded.
Meanwhile, London ended up with all of the gold and silver that should have gone directly to the Indians in exchange for their exports.