An alert driver who is planning ahead and driving at a suitable speed for the road and traffic conditions should have plenty of time to see a hazard and reduce speed safely or even bring the car to a gentle stop. Awareness, anticipation and planning are the key to avoid sudden braking or performing an emergency stop.When learning to drive or during the driving test your instructor or examiner will ensure that it is safe to perform the simulated emergency stop by checking that there is no one following close behind.
There are, of course, some occasions when even the best of us get caught out by a sudden danger, such as a vehicle or child crossing our path and it is entirely reasonable to perform an emergency stop. If the following driver hasn't left a sufficient gap and cannot stop in time then it can be successfully argued that they could have avoided the collision had they done so. However, slamming on the brakes for no apparent reason is clearly potentially dangerous and the Court would look at the reasonableness of doing so.
When it comes to making an emergency stop for an animal it would appear that reasonableness is based on the type and size of the animal. It may be deemed more reasonable to perform an emergency stop for an animal large enough to cause damage to the vehicle or injury to the occupants and by law you must report hitting some types of animals, as specified in the Highway Code, such as, dogs; horses; cows or sheep.
A Court may deem it unreasonable to perform an emergency stop for a small animal or bird but as any reaction could simply be instinctive they will look at each case individually and it could again be argued that the following driver could have avoided the collision.