You can’t be told to take annual or sick leave to cover for a downturn in business caused by COVID-19. If your employer tries to make you take annual or sick leave make it clear that they cannot legally do so.
You should use sick leave only if you are sick. Sick leave is only to be used in the circumstances described in s65 of the Holidays Act: the employee, their spouse or partner, or person who depends on the employee is sick or injured.
You can agree to take annual leave - but you actually have to be on leave. They can’t keep you working in any capacity.