I'd presume most workers are members, but they have ~5 million members and about 56k employees, so even if all of them are members they amount to ~1% of the votes.

EDIT: In the UK, the most prominent retail workers coop of sorts is the John Lewis Partnership. It has more employees than Co-op, at ~80k (in addition to John Lewis and Peter Jones it owns Waitrose), and it could be argued that in some senses it may not strictly be a workers coop - it's owned by a trust for the benefit of the workers of the business so its employees does not have the same direct say in the operation as a pure workers coop - but the terms of the trust makes it somewhat close.