By Agence France-Presse
The operation, which involved 180 officers in Milton Keynes, north of London, targeted an organised crime group that police believe was involved in trafficking, slavery and drugs.
Belgian officers helped Britain's immigration and police investigators.
"This group is using hand car washes as a front for their criminal activity and we believe individuals may be being trafficked into Milton Keynes, and might be the subject of modern slavery," police said in a statement.
Officers also carried out raids at 20 residential addresses.
"We have seized a large quantity of cocaine, cannabis and also thousands of pounds worth of cash, and a Mercedes ML 250," added the statement.
A total of 15 arrests were made during Tuesday's raids, while 30 suspected victims of slavery were "safeguarded."
The operation was sparked by the arrest in Belgium of an alleged people smuggler on Tuesday morning, according to the public prosecutor's office in the city of Bruges.
He was accused of being in the process of trafficking six fellow Albanians to the UK, including a 16- and 17-year-old boys.
"This is the largest operation of its type carried out within the Thames Valley," said British police detective Darran Hill.
"As well as working with partner agencies and charities, we have also worked closely with the National Crime Agency, and the Belgium Federal Police," he said.
Britain's crime agency last month launched an appeal to tackle modern-day slavery, urging people to report suspicious activity.
A report last year found that 3,805 people were referred as potential slavery victims in Britain during 2016.
Among the more regular victims are women and girls trafficked to work in UK nail bars for no pay.