The notice period that an employee must give should be set out in their contract. When someone resigns you should ask for their resignation in writing. Please confirm with them the last day of their employment and whether there is any outstanding annual leave for them to take before that date.
Set a date for an exit interview and ask the employee for their feedback. The idea is to enable them to leave positively and for you to learn if there is anything you could improve.
Redundancy
Sometimes redundancy is necessary. There are rules about how to do this fairly and ACAS sets these out here: Redundancy | ACAS.
Managing volunteers
Volunteer Centre Tower Hamlets has many excellent resources for managing volunteers: Managing Volunteers - Resources - Volunteer Centre Tower Hamlets. They also offer training, a bulletin and 1-1 support free to any voluntary and community group in Tower Hamlets.