Great summary, thank you for posting that.
I think it is reasonable to assume the driver had knowledge of events on the train through passcoms (BBC have reported there were ‘alarms’ going off in the ) which may have been a little before 999 calls were logged. The driver would then have been on to Huntingdon workstation straight away, whilst braking for the turnout at Huntingdon North junction for which the signalling was not yet set.
Absolutely commendable action by the driver a) for pressing yellow rather than red, b) for knowing exactly where he/she was and that Huntingdon was possible and the best option, and c) working with the signaller to make that happen swiftly.
* On most trains when the passcomm is activated a specific alarm sounds in the cab. If the driver makes an announcement (some reports suggest announcements were made during the incident, acknowledging what was happening and that staff were doing something about it), then the alarm will be audible in the background.
EDIT: whilst I was posting, news of the member of LNER staff who tried to stop the sole attacker has emerged. Commendable and very courageous action by him as well, of course.