I work in one of the aforementioned boroughs where CPV's are used, and to good effect. In defence of the process, it is not a blanket referral and you gfet a volunteer system, every allocation to a family is carefully matched and in some circumstances it is not appropriate for a volunteer. Rsponsibility does not shift from the social workers, they remain responsible for the risk assessment/management, not the volunteer. The volunteer is there to support the family and not take on social work roles. CPV's seem to take on the role of the old fashioned 'family care worker'. Stat visits must still be undertaken by the allocated worker.
I have found, where i have had a CPV allocated, for them to be effective. You dont need a DipSW or BA to be a good enough parent or support someone else with their parenting and that is their role.