Population in Custody: Monthly tables / Ministry of Justice, 31 July 2008
http://www.justice.gov.uk/publications/populationincustody.htm"This is a monthly statistical release presenting tables on the population in custody, with summary figures on the population in prison establishments, police cells, secure children's homes and secure training centres.

Update:
The tables for June 2008 have been added.
PDF - information on statistics
http://www.justice.gov.uk/docs/population-in-custody-jun08.pdf
Tables June 2008 http://www.justice.gov.uk/docs/population-in-custody-jun08.xls