this would be an awful lot to put in jail, don't you think
wiki
Although it is difficult to know how many people reside in the UK without authorisation, a Home Office study based on Census 2001 data released in March 2005 estimated a population of between 310,000 and 570,000.
More recently, a study carried out by a research team at LSE for the Greater London Authority estimated the undocumented migrant population of the UK by updating the Home Office study. The LSE's study takes into account other factors not included in the previous estimate, namely the continued arrival of asylum seekers, the clearance of the asylum applications backlog, further undocumented migrants entering and leaving the country, more migrants overstaying, and the regularisation of EU accession citizens.
The most significant change in this estimate is however the inclusion of children born in the UK to undocumented migrants. For the LSE team undocumented migrants oscillate between 417,000 and 863,000, including a population of UK-born children ranging between 44,000 and 144,000. Drawing on this and taking stock of the outcome of the recent Case Resolution Programme, a University of Oxford's study by Nando Sigona and Vanessa Hughes estimate at end 2011 a population of undocumented migrant children of 120,000, with over half born in the UK to parents residing without legal immigration status.