If you're unhappy with the quality of service you've received at an optician, you should make a complaint to the practice. The practice should have a complaints procedure, which you're entitled to see.
It's always preferable to settle a complaint between the practice and yourself. If it can't be resolved and relates to a NHS eye examination you received, you can take your complaint to your local NHS Board. The NHS Board will deal with the complaint in accordance with their own complaints procedures.
If your complaint relates to glasses or contact lenses you received, this is a private transaction between you and the optician. The NHS has no role in investigating any such complaints. You could contact the Optical Consumer Complaints Service, who are an independent body set up to try to settle complaints about the quality of service received.