"It's extremely difficult to get asylum in the UK, especially if you are not physically in the country," said Motley. "I think by them taking him back to the UK it is a recognition that there was a legal error that took place and that they are trying to correct."
Barr said the UK government's decision to deport a Sikh to Afghanistan was "shocking" given the country's limited religious freedom.
"Religious minorities are a very small portion of the population in Afghanistan, and are sometimes tolerated and sometimes not tolerated. So for the UK to send him back in the first place without carefully considering the situation of Sikhs in Afghanistan and the treatment that would await him is shocking," she said.
Singh told the Guardian by phone from Birmingham, where he is now seeking asylum, that being there felt "unbelievable". He had asked that his case not be publicized until he was back on British soil, because of worries it could complicate his departure.
"I never thought I would see the UK again … So many people are still stuck in jail [in Afghanistan]. I am so lucky," he added.
The British embassy declined to comment on the details of Singh's case, but suggested that it was convinced he is Afghan, and he was returned to the UK only because that could not be proved.
"Individuals are only returned to a country when there is substantial evidence that it is their country of origin," an embassy spokeswoman said. "We have agreements in place with certain countries that mean we will re-admit individuals unable to prove their nationality to the satisfaction of the receiving country's authorities."
The Afghan justice ministry said it was not aware of the case, and the attorney general's office did not respond to requests for comment.