Didn't the driver at Potter's Bar only know something had happened because the brakes came on?
IIRC he felt a series of jolts, which was very shortly afterwards followed by the emergency brakes coming on. However he wasn’t aware it was a derailment until looking back along the train.
The report doesn’t actually say, but the fact that he looked back rather than commenced fault-finding on board the train suggests he had quickly formed the opinion something serious was wrong.
There was also a Railtrack manager in the cab who observed a signal briefly revert to danger, which may have been a factor. Again the report isn’t entirely clear, however it seems the driver didn’t notice this, presumably he might have been looking at cab indications to see if anything was indicated that could explain the jolts or brake application. All this would of course have happened within a couple of seconds.