Operation Grange is overseen by a Gold Group. Such groups can include representatives from various other agencies. Operation Task, for example, was led by the ACC of LC and included representatives from the CPS, FCO and the Home Office. Policing representatives were from ACPO, CEOP, MPS, MPIA and the SIO.
The debrief of Operation Task, carried out in 2009, says the group developed a strategy. One of their aims was to "secure and preserve evidence which may assist in the investigation of her abduction"
http://library.college.police.uk/docs/npia/Strategic-debrief-operation-task-2009.pdf (Page 9)
So Operation Task appear to have decided what crime had been committed. I'm not sure what 'securing' evidence means, but it seems they were interested in a specific type of evidence. That which "may assist in the investigation of her abduction"
Given the above, it's hardly surprising that Operation Grange appear to have restricted their investigation to abduction; that opinion was formed as long ago as 2007 in the UK, it seems.