A certificate for Burial or Cremation (known as the green form), unless a coroner has given you an Order for Burial (form 101), or a certificate for Cremation (form E), give permission for the body to be buried or for an application for cremation to be made.
The law does not impose a duty on anyone to use an undertaker to carry out a funeral. Indeed the law does not impose a duty on anyone to hold a funeral and some people choose to donate their bodies to science. In most circumstances, the green form is given to the “person procuring the disposal”. That person is usually the nearest relative. They are then legally required to give it to the “person affecting the disposal”. That person is whoever is responsible for the burial place or crematorium. It is stated very clearly on the back of a green form, a Coroners Order for Burial (form 101) or a certificate for Cremation (form E) that they “must” be given to the person responsible for the burial place or crematorium. That always applies, apart from one very unusual circumstance about English and Welsh churches. If visitors to this site require a more technical explanation about this do ask.
This means that either “form” must be given to the land owner or person responsible for cremation. The law requires that the form be given to the person responsible for the burial place (which could be a garden or field) or the crematorium.