Sentencing decisions are a matter for the independent courts.
When deciding what sentence to impose the courts take into account all the facts of each case. This will include the seriousness of the offence, the culpability of the offender and aggravating and mitigating factors, in line with any relevant sentencing guidelines issued by the Sentencing Council, which cover England and Wales.
The Ministry of Justice publishes criminal justice statistics, including sentencing and average custodial sentencing length, on a quarterly and annual basis. The latest quarterly statistics, to June 2018 are available at https://www.gov.uk/government/statistics/criminal-justice-system-statistics-quarterly-june-2018