The Home Office collect and publish data on the number of arrests for notifiable offences, by police force area, on an annual basis. Data can be found in the ‘Police Powers and Procedures’ statistical bulletin and accompanying tables, the latest of which can be accessed here: https://www.gov.uk/government/statistics/police-powers-and-procedures-england-and-wales-year-ending-31-march-2016
The Home Office does not hold information on the number of arrests that led to a prosecution or conviction. Data on prosecutions and convictions are the responsibility of the Ministry of Justice. Data on arrests cannot be linked to prosecution and conviction data.