I'll tell you now, the police will push not to have the Army involved, reason being it will undermine their position.
The only time I've been involved with civil disorder in the UK was back in 1984, when the first cruise missiles came to Greenham Common.
At the time they had a police cordon around the base. The night before we were deployed into the base - 600 paratroopers - in case the police couldn't contain the peace protesters ? Well, it kicked off, the rioters went through the police cordon like a hot knife through butter. Then they came face to face with us, we batter them and stop them in their tracks. For the whole two weeks we were there, not a squeak from them.
The End.