Within English law an ATCO would owe a duty of care to those to whom he provides a service and could be liable in negligence. Liability depends on their being a duty of care, breach of that duty and actual damage (e.g. personal injury).
Employers are generally liable for the torts of their "employees" when the employee is acting within the course of his employment.
The Crown Proceedings Acts date from 1947 and have generally made the Crown liable for the torts of Crown servants (e.g. military ATC) provided that a non-Crown servant would be liable in the same circumstances.