I notice there was mention of Thatcham and Bedwyn.
Thatcham. If the train is in the correct formation leaving london, the driver at Thatcham could if he wished take the power car off the platform at Thatcham in the down direction and stop just on the level crossing. The signal would not go back to red and the TGS coach A will be platformed Some drivers do insist on stopping at the signal when it is clear as they do not know it will not go back to red as the track circuit for this signal is past the level crossing. If the signal is red when the train approaches and does not clear the driver has no other option than to stop at the signal.
Bedwyn. Down direction - no matter what formation the train is in the power car must stop before the end of the platform where the signal is because the terminating Bedwyn services (turbos) shunt into the sidings and the cross over for this movement is just beyond the signal. Bedwyn does not have an overlap.
I know there are 2+7/8 stop-boards at Thatcham which are the other side of the level crossing, and i'm 99% sure there are also boards at Bedwyn after the signal, so why would they be provided if they are not to be used?
As another example, Exeter St. Thomas on the Up Main has a platform signal which also has no over-lap, yet drivers stop at the stop-boards after the signal and the signal of course will revert to danger --
TM‡'s have to check as the train arrives the signal is proceed before the driver passes it, as the signal will be red when dispatching (train of course in next section).
Interestingly I had a conversation with a driver comp manager from Paddington yesterday, who would expect drivers to stop at the boards at Thatcham and Bedwyn if first class is leading and signal is clear - however if a clear understanding is reached to differ from this, then that's up to the crew.
Either way, there is no signal before the stop boards involved in the Twyford scenario, so i'm still at a complete loss as to why this happened - especially as the TM knew 5 mins prior to arrival.