Complaints must be in writing and addressed to the branch where you picked up the car. This is because the actual contract is signed with whichever hire car company gives you the car. Even if you have booked your rental car before your holiday, the booking agency is only liable for its own actions. In many cases these agencies nevertheless help consumers with any subsequent issues.
If a car rental company charges your credit card for services that you have not asked for, it is also worth writing to your credit card company and asking for a refund (Consumer Protection Act, Chapter 7, Section 39). If possible, accompany your complaint by a copy of the contract and any correspondence between yourself and the car rental company.
If the car rental company fails to respond to your complaint or if the matter remains unresolved otherwise, you can contact the European Consumer Centre.