You can complain directly to the funeral director.
If they are a member of a trade body, you can complain to them.
The scheme for funeral directors who are members of The National Association of Funeral Directors (NAFD) is with an independent arbitration organisation called the Centre for Effective Dispute Management (CEDR). The scheme is funded by the NAFD and is free to the public.
NAFD: https://www.nafd.org.uk/standards/nafd-resolve/
The scheme for funeral directors that are members of the National Society of Allied and Independent Funeral Directors (SAIF) is the SAIF Consumer Protection Scheme. The scheme is run by SAIF’s professional standards committee and is free to the public.