I assume that was a tongue in cheek question, but if not... 'essential staff' are without whom NATS could not provide an ATC service (ATCOs, ATSAs, Engineers).
'Non essential' staff are those who support the business in other ways - project staff, HR, management etc...
At times like these NATS management remembers what ATC is actually about - at other times 'essential staff' are seen as a pain.