Beyond reaching out directly to the company in question, ‘complaints’ on their own only really have power if they are made in accordance with a regulatory framework that provides additional enforcement, such as the Financial Ombudsman Service. If the firm you have an issue with would not fall within the scope of an independent adjudicator such as the Ombudsman, you may only have recourse through the police or through a civil action in the courts. In which case you may need a solicitor to advise you of your options. I would recommend speaking to consumer rights groups like Citizen’s Advice to see if they could suggest any other free routes to exhaust before turning to the more costly options.