I was under orders....
... didn't work out so well during the Nuremburg trials and there refusing an order got you shot ( I know not UK law but atleast a precident)
"to the best of my knowledge" clearly doesnt not apply either since they stopped the investigation for a reason.
Personally I see this as just another example of law only applying to the masses (assuming the police can be bothered) and none for the elite, we will have to wait for the final outcome since policy causing people to commit suicide might put the current government in the dock and they are definately not going to say "it's a fair cop Gov'ner"