www.biduk.org is the website of BID - Bail for immigration Detainees. This organization supports detainees to make their own representations at bail hearings. The website gives links to research papers, press releases and other publications specifically about immigration detention.
Detention Action campaigns on immigration detention and supports individual detainees. http://detentionaction.org.uk/
The Migration Observatory provides general information, research papers and statistics on many areas including detention and deportation: http://www.migrationobservatory.ox.ac.uk/