An interview is just one part of the overall investigation. Any police investigation should be looking at the whole picture.
The role of the police is to establish evidence and refer the matter to the court (via the CPS) unless a full admission is made and then consider a caution etc.
A lot of people (including some police officers and solicitors) seem to think that the role of the police is to decide guilt of the person arrested. In actual fact it is to gather as much evidence as possible - from all angles - in order to establish the strength of evidence against all parties and to decide if there is enough evidence for a charge / caution or indeed an NFA.
The same amount of supporting evidence is needed for a caution as for a charge so be wary of solictors whose advice around what to say in interview is based purely around whether you are likely to get a caution or not.