Well there isn’t a head copper. There is this which may interest you
https://www.npcc.police.uk/NPCCBusinessAreas/TerrorismandAlliedMatters.aspx
That man’s rank is junior to the mets chief commissioner.
There are territorial constabularies and also private police forces.
The incident location would dictate which constabulary would in effect deal with it but mutual aid would be very probable for smaller shire constabularies.