According to the gradient profiles book there is no such point: the Lickey is 37¾ all the way to Blackwell, where the line then becomes almost level. So the load against gravity begins to reduce only when the loco at the front passes Blackwell. Without some net assistance from the banking engines (i.e. enough force from one or both of them to lift them themselves up the bank with some left over to push the back coach) the TE of the Jubilee alone would not have been able to accelerate the train until that loco and some of the coaches had passed Blackwell. The account seems clear that the acceleration started before that. The long distance between the train engine and the bankers and the long time since the event both prevent us from knowing the sequence of events for certain, but I can only continue to endorse Tom's arguments from physics (which I think I'm entitled to do, as a physics PhD).