The legal distinction between an employee and a volunteer is complex given the differences between the types of voluntary work and volunteering activities that individuals can be involved in. The principal difference is that volunteering must not amount to unpaid work or job substitution. In particular: there should be no payment, other than reasonable travel and meals expenditure actually incurred (not an allowance for the same); there should be no contractual obligations on the volunteer; the volunteer is helping a registered voluntary or charitable organisation (or organisation that raises funds for either); the volunteering is for a public sector organisation; and the volunteering is not a substitute for employment (i.e. fulfilling a role that a salaried worker would normally fulfil).
Although it is Home Office policy to support asylum seekers volunteering for charities or public sector organisations; we make it clear that it is the responsibility of any organisation considering recruiting volunteers to ensure that such activity does not constitute voluntary work or employment. Further guidance is available on gov.uk: https://www.gov.uk/government/publications/handling-applications-for-permission-to-take-employment-instruction