Three Afghan asylum seekers who were taken to London from an overcrowded migrant processing centre in southeast England voluntarily returned to the site on Wednesday after being left with nowhere to stay, according to a Reuters photographer.

The British government is under pressure over conditions at the site at Manston in Kent, which is operating at more than double its capacity, with people staying much longer than the intended 24-hour period and sleeping on mats on the floor.

Immigration minister Robert Jenrick said the numbers at the site had fallen "very substantially" since the weekend.

Two British newspapers reported on Wednesday that a group of about 40 migrants from Manston were dropped at a coach station in central London on Tuesday and while about 30 had family or friends they were able to contact to stay with, 11 were left with no accommodation and were helped by a charity worker.

They cited a British Transport Police spokesperson who said police had responded to reports of a group of asylum seekers looking for assistance at Victoria station late on Tuesday.

"Officers engaged and liaised with charity partners, rail staff, and government colleagues to help them find accommodation for the evening," the spokesperson was quoted as saying.

A Reuters photographer outside the Manston processing centre said three Afghan men who said they were taken to London on Tuesday but left with nowhere to stay returned to the site in Kent on Wednesday evening as they had no other shelter.

The Home Office, Britain's interior ministry, declined to comment on operational arrangements at Manston.

Home Secretary Suella Braverman has been accused of failing to listen to legal advice on prolonged detention at the centre, a claim she has denied. An independent inspector said one Afghan family last week told him they had been there 32 days.

Jenrick told Sky News on Wednesday the Home Office was facing legal action over the centre.

"I believe we have received the initial contact for a judicial review," he said, declining to comment on who had brought the challenge. He added that he expected the site to return to being legally compliant "very rapidly".