The structure for policing is laid out in:
 

The Policing Protocol Order 2011
 

• The Police Reform & Social Responsibility Act 2011
   

• STATUTORY INSTRUMENTS 2011, No.2744
  
  Please see the below Gov.co.uk legislation:-
  https://www.legislation.gov.uk/uksi/2011/2744/made
  
  This enables the Home Secretary "backstop" to override the PCC and Chief Constable in public interest and national security to take control

. ... Lifted word for word from the Gov website:-

28.  The Home Secretary is ultimately accountable to Parliament and charged with ensuring the maintenance of the Queen’s Peace within all force areas, safeguarding the public and protecting our national borders and security. The Home Secretary has reserved powers and legislative tools that enable intervention and direction to all parties, if it is determined by the Home Secretary that such action is necessary in order to prevent or mitigate risk to the public or national security. Such powers and tools will be used only as a last resort, and will not be used to interfere with the democratic will of the electorate within a force area, nor seek to interfere with the office of constable, unless the Home Secretary is satisfied on the advice of Her Majesty’s Inspectorate of Constabulary that not to do so would result in a police force failing or national security being compromised.

29.  The Home Secretary retains the legal accountability for national security and the role that the police service plays within the delivery of any national response. The Home Secretary has a duty to issue a Strategic Policing Requirement that sets out what are, in her view, the national threats at the time and the appropriate national policing capabilities that are required to counter them.

Operational Matters

30.  The operational independence of the police is a fundamental principle of British policing. It is expected by the Home Secretary that the professional discretion of the police service and oath of office give surety to the public that this shall not be compromised.

31.  The current arrangements are defined in part by the 2011 Act.

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• The Police Act 1996, sections 40a and 40b
  
  https://www.legislation.gov.uk/ukpga/1996/16/section/40A
  
  F140A  Power to give directions in relation to [F2local policing body]
  
  (1) Where the Secretary of State is satisfied that a [F2local policing body] is failing to discharge any of its functions in an effective manner, whether generally or in particular respects, he may direct the [F2local policing body] to take specified measures for the purpose of remedying the failure.
  
  (2) Where the Secretary of State is satisfied that a [F2local policing body] will fail to discharge any of its functions in an effective manner, whether generally or in particular respects, unless remedial measures are taken, he may direct the [F2local policing body] to take specified measures in order to prevent such a failure occurring.
  
  (3) The measures that may be specified in a direction under subsection (1) or (2) include the submission to the Secretary of State of an action plan setting out the measures which [F3the local policing body] submitting the plan proposes to take for the purpose of remedying the failure in question or (as the case may be) preventing such a failure occurring.