//You do not have a contract of employment as a volunteer, so you do not have the same rights as an employee or worker.
You will usually be given a volunteer agreement that explains:
•the level of supervision and support you’ll get
•what training you’ll get
•whether you’re covered under the organisation’s employer or public liability insurance
•health and safety issues
•any expenses the organisation will cover
The volunteer agreement is not compulsory, but sets out what you can expect from the organisation you’re volunteering for. It does not form a contract between you and the organisation.
The National Council for Voluntary Organisations (NCVO) has information on volunteers’ legal status...//
https://www.gov.uk/volunteering/volunteers-rights
I think you can take a tea break or lunch break if you choose to. As a volunteer you're not obliged to do anything you don't want to do.