The haul of 8.4 million doses of medicines included potentially harmful slimming pills, controlled drugs such as diazepam, anabolic steroids and anti-impotence pills.
The Medicines and Healthcare Products Regulatory Agency (MHRA), which was responsible for seizing products worth 8.6 million pounds, said on Thursday that the Interpol-coordinated operation was conducted between May 11 and 21.
The crackdown also targeted 10,603 websites, leading them to be closed down or suspended through having their domain name or payment facilities removed.
"The medicines recovered during these raids were being held in appalling conditions, such as a dirty old building with broken windows, with medicines lying on the floor in bin bags," MHRA Head of Enforcement Alastair Jeffrey said in a statement.
"Criminals involved in the illegal supply of medicines through the internet aren't interested in your health; they are interested in your money."
India was the source of 72 percent of the illicit medicines seized in Britain, while China accounted for 11 percent, the MHRA added.