so, you saw a plane flying along in the rain, alongside a major road.
I've been flying along the road in rain and still been in VMC having INFLIGHT visibility better than 3 miles.
IF you were sure the plane was in some violation of safety regulations, I suggest you contact the CAA (isn't that the equivilent of the FAA in England)along with their registration number and time/location.
Then the proper authorities can take proper action after a proper investigation. AFter all you only estimated the height (we might say altitude).
I once reported someone for getting an instrument clearance without an instrument rating...the FAA investigated and took them to lunch, telling them not to do that again.
Just think, someone could get a free lunch if you call in!