having seen first hand how the CAA would drag a person throught the Crown Court system and spend over £400,000 on heresay only to drop the charges after 2 years of preparation and 8 days in court, I would not report anyone to them.
I would drop the owner a nice polite note to inform him that he or whoever was using his aircraft seemed to be a bit low close to the motorway and that some "anti" might report him.
Dont give yourself a heart attack over it, in my experience if a person flys low continually he is a "temporary" problem and we will soon be reading about him in 1 of our mags.