I believe that you will find that it is in hand. You may recall that the PM appointed Adml West as security supremo for just the purpose of protecting the country from an enemy within.
It is true that the armed forces role is to defend UK and also our interests overseas. At home however the armed forces are subordinated to the civil authorities. In requesting military aid to the civil power primacy rests with the civilian authorities until such time as it is officially transfered - viz Ultimate Force
The last thing I would want, from either side of the fence, is a squadie with a machine gun on a bridge.