The Home Office has paid a handful of private contractors hundreds of millions of pounds to run the UK’s immigration removal centres, but no one knows for certain just how profitable the industry is. https://www.theguardian.com/uk-news/2018/oct/10/private-contractors-paid-millions-uk-detention-centres-some-firms-making-30-percent-profit

One of the 10 UK facilities is run by Her Majesty’s Prison and Probation Service, but the rest are contracted out to outsourcing firms G4S, Mitie, Serco and the US-owned GEO Group.
[MULTIPLE ISSUES OF ABUSE BY G4S AT YOUNG OFFENDERS AND IMMIGRATION CENTRES]

Commercial confidentiality agreements mean the Home Office and outsourcing companies are not obliged to publish detailed financial information about immigration detention centres in the UK. - IT ALL SOUNDS AWFULLY FAMILIAR, IRELAND....

But figures released under FOI and published on the government’s Contracts Finder website show the value of some contracts when they were awarded.
[In 2018] Mitie won what is believed to be the largest immigration detention contract ever awarded, valued at more than £500m.

The Mitie contract covers a range of services and it is not known how much of this is for management of removal centres. But they run Larne House, Northern Ireland's removal centre, as well as others.

Larne House is a Short Term Holding Facility where people here are held for max 7 days before either being transferred to one of several Detention Centres (or IRCs) in Great Britain, deported or released. Up to 19 people are detained in former prison cells of a police station.

Larne House Visitors’ Group is a member of the Association of Visitors to Immigration Detainees. Heiko Topp who coordinates the group says; “People enter immigration detention in Larne and then face the prospect of indefinite detention when transferred to GB. [...]

"The Home Office consistently ignores its own guidelines and detains pregnant women, children and survivors of torture. The UK is the only country in the European Union which does not have a time limit on depriving people of their freedom." Heiko Topp, Larne House Visitors Group.