Depending on the circumstances, hiding the bodies would be either Obstructing a Coroner or Preventing the Burial of a Body
Any disposal of a corpse with intent to obstruct or prevent a coroner's inquest, when there is a duty to hold one, is an offence. The offence is a common law offence, triable only on indictment and carries a maximum penalty of life imprisonment and/or a fine.
The offence of preventing the burial of a body (indictable only, unlimited imprisonment) is an alternative charge. Proof of this offence does not require proof of the specific intent required for obstructing a coroner.
Burning the van could be either Criminal Damage by Arson or perverting the course of justice, again depending on the circumstances. The latter is committed when an accused:
does an act or series of acts;
which has or have a tendency to pervert; and
which is or are intended to pervert;
the course of public justice.
The offence is contrary to common law and triable only on indictment. It carries a maximum penalty of life imprisonment and/or a fine. The course of justice must be in existence at the time of the act(s). The course of justice starts when:
an event has occurred, from which it can reasonably be expected that an investigation will follow; or
investigations which could/might bring proceedings have actually started; or
proceedings have started or are about to start.