The HMRC guidance on payments to volunteers states:
"Genuine volunteers do not expect, and are not entitled to receive, any monetary payments or benefits in kind for their volunteering duties. Any payments made to volunteers (particularly if they are made regularly), expenses greater than can be realistically incurred, or entitlement to benefits in kind could indicate that the person is not a volunteer, even if they think they are, but is actually a worker and will be entitled to the national minimum wage.
However, a genuine “honorarium” (a gift of money) in the form of a one-off gift which the volunteer had not expected to receive, with no obligation and of a reasonable amount is not likely to give the volunteer the right to the minimum wage."
So a one-off payment should be OK, but annual may not be.