I work for the City Council in Bristol and we regularly prosecute people for owning pitbulls. The police here also prosecute for owning pitbulls. As a result of the prosecution the magistrates will order a contingent destruction order giving the owner a certain length of time to do certain things. This includes neutering, chipping, insuring. If the dog goes out in a public place in must be muzzled. If the owner does not do these things then the dog will be put to sleep. This assumes that the dog isn't dangerous of course, not all of them are. The ownership of the dog cannot be transferred. So in my area yes action would be taken but in other areas I don't know. If you did report it, it would not mean that the dog would automatically be put down.
Hope that helps
