UK isn't cooperating with India when it comes to the extradition of Vijay Mallya, Nirav Modi and Lalit Modi who committed financial fraud.
Actually, UK Government drafted a MoU regarding the deportation of illegal migrants in the country. This is a measure to take action against around 75,000 illegal migrants (most of them Indians) in UK. PM Narendra Modi didn't sign this pact during his visit to UK in April this year.
UK Minister of State for Countering Extremism Baroness Williams discussed about Illegal Migrants with Indian MoS Kiren Rijiju on Monday. A Message has been conveyed that UK wouldn't help in the extradition of Mallya, Nirav and Lalit if India doesn't sign the deal. It's during this meeting, UK confirmed the presence of Nirav Modi in Britain.
Indian Government could be hesitating to sign the MoU over Illegal Migrants over concerns of mass deportation, security breaches and a consent clause.
In 2017, Close to 5,000 Indians staying in UK had voluntarily returned to India. Another 700 people were deported by the UK Government.