Not really possible on a single car forum. Works best for regional car sites.
Edinburgh Cruise has a Police Liason forum which is brilliant but the laws are different in Scotland and England (and probably that other place stuck to england)
Also The police help out on these forums for their local regions as it promotes better awareness in them and thus they are doing their job by helping out on the forums. They wouldnt be able to spend the time posting on a forum which covers the whole of the UK (and more)
They are a great adition to a regional forum, just not really possible on one like this im afraid.