Of course, I am not in Wales, but our isolation unit is our main building and adjacent paddock. For most of the year it is not used as an isolation unit, it is just part of the farm. A week before the first show I record what is in it. Sheep from the Isolation unit can then move to and from shows without triggering a standstill on the rest of the farm. If there is a long enough gap between shows, stock can move between the isolation unit and the rest of the farm as long as the unit is closed a week before the next show.
In the beginning, the isolation unit had a separate holding number - in the 6000 or 7000 rage if memory serves, so that the Trading Standards database was in no doubt that movements to and from shows were into and out of the isolation unit. About three years ago DEFRA abolished the separate holding number but your record on the database flags up the fact that the main holding has an isolation unit if a movement form is processed which appears to break the standstill. As long as the movement is to or from a holding number for a showground, no query is raised and you are left alone to manage the movements without any more red tape.
As Kol says, there is no restriction as to the age or purpose of stock in the unit and Trading Standards cannot possibly know from the AML1 form anyway.