In this instance, the police are simply a tool being used by the Tory government (again) to pursue a minor misdemeanor (at best). The police don't need to know WHAT they're investigating, the Tories just need people to believe that they ARE investigating.The Police don't even seem to know what it is they are actually investigating. CGT avoidance is an HMRC issue - so it can only be electoral fraud or council-tax evasion.
Over the next 6 months, there will be more and more red herrings thrown to the masses in the hope that'll be enough for some to draw the necessary false equivalences to vote blue in the next GE.