As I understand things, each regional brigade is meant to co-ordinate the military assets aiding the Civil Power. The Police have primacy, with the military component based around the resident TA Inf Bn. Each Bde HQ also has, for want of an alternative expression, a reinforcement team of watchkeepers etc to run it 24/7.
The Reinforcement Teams do 19 days a year, so it can be a "full time" TA appointment.
If others are better informed, please do correct me.