Each local Citizens' Advice Bureau is a separate registered charity, so (unless you were complaining about them breaching legislation relating to the administration of charities), there's no such thing as an 'official' complaint.
You can only write to the Chairman or Secretary of the relevant CAB, to express your concerns about something. Remember that three-quarters of all CAB staff are volunteers so, even with the best of training, some errors are inevitable.
Dear Curly Sue,
It will be difficult depending upon your complaint. If the secretary is not forthcoming you can go to the charity commission.or.uk
But to be honest you maybe just flogging at confused voluntary worker with dementia
,If your CAB is like mine it is run by well intentioned OAP's who confuse the heck out of the clients and themselves. Or course many are not like this ( he backpeddles furiously) ..Anyone go a ladder