Not 100% sure, but i am pretty sure that if the airfield wants to be licensed by the CAA, as opposed to little or no regulation, they also have to provide a minimum service to a/c during the period of time the licence is operational.
For example, the owner my want flying training or flying for hire and reward to take place at certain times, and this would be when a FISO or better service, and minimum fire sevice cover would be required.
I don't think that there is a minimum number of movements, it's just those that may be operated for hire and reward that count.