This information is not held centrally by the Home Office.
The Independent Police Complaints Commission (IPCC) publishes quarterly data on police complaints for each force, which includes a breakdown of the nature of allegations recorded. The category of ‘Discriminatory Behaviour’ includes incidents of racism.
The IPCC also holds a more detailed breakdown of the category information, but does not hold information of the disciplinary outcomes subsequent to the complaints. This information may be held by the force itself.