This is really an interesting thread. I will admit, I am not a pilot. I am a horse owner. I don't live in England, rather I live in the US.
Here we have laws covering such situations. They mainly pertain to cars and such, but one could certainly make a case for a low-flying helecopter as well. For instance (SS means State Statute):
SS#346.11 Passing or meeting frightened animal:
Whenever a person riding, driving or leading an animal which is frightened gives a signal of distress to the operator of a motor vehicle by a raising of the hand or otherwise, the operator of the motor vehicle shall promptly stop the vehicle unless a movement forward is necessary to avoid an accident or injury and shall, upon request, stop all motive power until such animal is under control.
Also, no person shall intentionally frighten an animal so as to endanger the animal and/ or handler.
SS#346.21 also defines the right of way laws for livestock which under the definitions of the statutes horses qualify.
The operator of a motor vehicle shall yeild right of way to livestock being driven over or along any highway, but any person in charge of such livestock shall use reasonable care and diligence to open the roadway for vehicular traffic.
(This obviously pertains to roadways)
This is such a sad story. I hope people learn something from it.
Condolences to her family.