Sector

Regulator INFORMATION ABOUT HOW TO COMPLAIN 
Water Industry  Ofwat / Consumer Council for Water  https://www.ccw.org.uk/advice-and-support/make-a-complaint/
Energy  Ofgem / Energy Ombudsman  https://www.energyombudsman.org/how-we-can-help/network-operators
Financial Services  Financial Conduct Authority  https://www.financial-ombudsman.org.uk/consumers/how-to-complain