Been away a few days and have not read all this thread - just the last few comments.

The British Army had an extensive history of deploying non-infantry units to Northern Ireland during "The Troubles", mainly from Germany (where we were deployed to face the USSR etc). This even included specialist units, armour, engineer, air defence and nuclear missile. I am sure there are articles in the British Army publications (British Army Review notably) on this theme.

Infantry units also had to be re-trained, for six month tours and several units did longer three year tours.

At one point, for several years wayback in the 1970's, the Army had primacy in security and law enforcement, as the police, the RUC, effectively collapsed and had little credibilty (a very short explanation). Today the RUC offer their COIN expertise around the world - not always without criticism.

At that time the Army deployed two very large military police regiments, to do "ordinary" policing and provide support to deployed army units. There are several, now dated published books on this period.

After a very long time the Army had a system for adapting to this COIN / support the police environment. I suspect non-infantry and artillery unit deployments declined.

I'll leave the use of police in COIN to another time.

davidbfpo