If you're a volunteer then you have no statutory employment rights (statutory - imposed by law). No pay, no holidays, no sickness rights, no maternity rights.
The only rights you may have would be as a visitor to the workplace - so health and safety - and human rights - so anti-discrimination.
There may be a contract between volunteer and the organisation which may set out the terms of attendance and notice. Any agreement can de ended by mutual agreement or disagreement.
Interesting article from Daily Mail on the
WRVS Sandwich Stasi.
Hope that helps.