People who are charged with immigration offences may be remanded in custody within the prison system but the majority of immigration detainees are housed in Immigration Removal Centres (IRC) on the assumption that they will be removed from the country within a short time.

In the years from 2009 to 2016, between 2,500 and 3,500 people have been detained in the UK at any given time. During 2016 28,900 people entered detention.
See http://www.migrationobservatory.ox.ac.uk/resources/briefings/immigration-detention-in-the-uk/

In 2015, 40,896 people were removed from the UK or left voluntarily after the Home Office initiated their removal. Of these, 5,238 were asylum applicants, or their dependents. See http://www.migrationobservatory.ox.ac.uk/resources/briefings/deportations-removals-and-voluntary-departures-from-the-uk/

Over recent decades the Home Office has operated a system of Detained Fast Track by which some people in the asylum system were ‘fast-tracked’ through their appeals process while in detention. In two court cases two fast-track processes (from 2005) have been declared unfair by the courts and people may ask for new appeals. Many, of course will have been returned to their country of origin. www.theguardian.com/uk-news/2017/jan/20/uk-high-court-rules-10000-asylum-seekers-treated-unfairly-detained-fast-track?CMP=Share_iOSApp_Other 

The detention of children was controversial in theory it should not happen any more although some are detained every year as age-disputed minors. www.refugeecouncil.org.uk/what_we_do/childrens_services/agedisputeproject

There have been criticisms of the conditions in Immigration Removal Centres, which are mainly run by private companies. The recent All Party Parliamentary Inquiry found that even the newly built centres were like prisons.

What has been built … is not just prison-like. It looks like a prison: harsh straight lines, built to high-security standards, bare of anything to soften the feel of the interior. It sounds like a prison – large echoing open wings. It feels like a prison: the attempts to call the places where the detainees sleep a ‘room’ is confounded by the fact that they are manifestly cells. The toilets have no seats, just a solid steel bowl. It smells like a prison: that toilet is inside the cell. In many cases, the detainees – who are prisoners, in any normal sense of the word – have to eat in those cells beside the toilet.
Church of England written evidence, quoted in The APPG Inquiry into the Use of Immigration Detention in the United Kingdom Report, pages 41–2.

Detainees are retraumatised with the noise between rooms, the sounds of screaming or keys jangling reminding them of previous experiences. Mental health deteriorates and there is a high risk of suicide. In 2015 Channel 4 News exposed abusive behaviour by staff in Yarl’s Wood IRC in England.
‘Yarl’s Wood: undercover in the secretive immigration centre’, 2 March 2015. https://www.channel4.com/news/yarls-wood-immigration-removal-detention-centre-investigation

Campaigners have asked for better oversight and human rights groups regard it as especially important that vulnerable people such as children, pregnant women, and the victims of torture or mental illness should not be held in detention.

A House of Commons all-party Immigration Detention Inquiry summary in 2015 recommended that detention should not be used so frequently, that there should be a time limit of 28 days for anyone held in immigration detention, that people should only be detained in order to effect their removal from the UK. The politicians felt that the UK should learn from the alternatives to detention already in use in the UK and elsewhere.

Learn more:
Migration Observatory briefing Briefing Immigration Detention in the UK, September 2016
All Party Parliamentary Group on Refugees & the All Party Parliamentary Group on Migration The Report of the Inquiry into the Use of Immigration Detention in the United Kingdom 2015
The Detention Forum is a network of organisations working together to challenge the UK’s use of detention.
Medical Justice publish reports on health issues for detainees.