If anyone suffers from insomnia, JDP02 is always useful;
https://www.gov.uk/government/uploads/.../defenceconrtibution1.pdf
Ch 2, Sects I and VI may be of interest.
SECTION I – MILITARY AID TO THE CIVIL AUTHORITIES
Introduction
203. Military operations in the UK, under Military Task (MT) 2.1, are placed under the overarching title of MACA. This section examines the principles, procedures and essential features of providing military assistance.
204. MACA is sub-divided into Military Aid to other Government Departments (MAGD), Military Aid to the Civil Power (MACP) and Military Aid to the Civil Community (MACC). These operations are distinct from one another legally and politically, as well as in terms of military implications. More detailed information on MAGD, MACP and MACC is provided in Chapters 3, 4, and 5 respectively.
SECTION VI – FUNDING ARRANGEMENTS FOR MILITARY AID
TO THE CIVIL AUTHORITIES
General
266. For the reasons outlined above, MACA activity is, with a few specific exceptions, not funded within the Ministry of Defence (MOD) budget and is, therefore, conducted on a repayment basis. Treasury rules dictate that Government Departments charge for services that do not form part of their funded tasks. No matter how valid a request for assistance may appear, Defence funds are granted for Defence purposes. If the cost is not applicable to Defence, then it represents an improper use of resources and must be recovered. 79