I agree that walking away isn't really an option when someone is repeatedly hitting you. I was talking about walking away at an earlier stage to reduce the risk of confrontation- in this case maybe once your neighbour was annoyed at you washing the car on a shared drive maybe you could have retreated at that point.
Knocking out your neighbour (if that means making him unconscious) may not be regarded as reasonable force.
I'm not clear whether this happened on your property or on the shared driveway.