I guess it's worth knowing that therapists in the U.K. aren't licensed - they may or may not belong to a professional body but it's not necessary to practice. The exception would be if he's a clinical psychologist where he needs to be registered with the HCPC, who would investigate complaints and decide whether he was in breach of his duty and sanction or dergister him. If he was deregistered he wouldn't be able to practice as a psychologist. If he's a counsellor or psychotherapist he would likely need to be registered, and accredited, to work in the NHS but he could work privately without either.

The registration process is different to licensing which is a US requirement. I say this because, if you do decide to complain, its worth knowing that different bodies can do different things in terms of sanctions.