Lord Feud has given some detail around the treatment of voluntary work under Universal Credit(which are slightly more generous than those under Job Seekers Allowance)-
'For Universal Credit claimants, advisers will be able to reduce a claimant’s expected hours of worksearch and work preparation activities by up to 50 per cent in order to accommodate voluntary work. However the claimant can continue to do as much voluntary work as they feel they are able to, as long as they continue to meet their other work-related requirements.'