It is best to start by talking to the people who were involved with your care or treatment raise your concerns directly with them in accordance with their local complaints procedure.
If this doesn’t help or if you don’t want to deal with it in this way, you could speak to your Local Authorities Trading Standards Team or to your local Environmental Health Team. If the practitioner is registered doctor, dentist or registered nurse etc. You can make a direct referral to the practitioner’s professional regulator (such as the General Medical Council, the General Dental Council and the Nursing and Midwifery Council etc).