There are a number of possible factors which may have contributed to the fall in the number of arrests since the year ending March 2008. Inspections carried out by HMICFRS in 2016, as part of their police effectiveness assessments, did not identify one single cause (PEEL report on police effectiveness).
One possible reason linked to the fall in the number of arrests is the increased use of voluntary attendance, where an individual attends voluntarily at a police station or at any other place where a constable is present without having been arrested for the purpose of assisting with an investigation. It is thought that the use of this practice has increased due to a more stringent application of the necessity test (which was introduced in 2012), where, for an arrest to be lawful, there must be reasonable grounds for believing that the arrest is necessary (PACE Code G).
There is also evidence of greater use of other outcomes, such as community resolutions, as part of efforts to reduce the number of young people entering custody. Other factors that may have had an impact on the fall in the number of arrests are discussed in the HMICFRS PEEL report 2016. See http://www.justiceinspectorates.gov.uk/hmicfrs/wp-content/uploads/peel-police-effectiveness-2016.pdf