You are effectively proposing what alegatta proposed previously. As you say, this kind of over-volt trip on the incoming main high-voltage power supplies would need to be faster and more sensitive than that on all the down-stream internal electronic power supplies - no point in switching after those have shut down. This would be technically very difficult, to say the least. Worse, such a sensitive trip would likely operate quite frequently, leading to the supply being far less reliable than it is now.
So, you would prevent the "once in a lifetime" shutdown that was experienced. But what you would have replaced it with is something that causes far more frequent momentary power-losses. As you say, the relay interlocking and remote control systems will restart by themselves after these momentary power-losses, but they will all still have led to the relay interlocking replacing all signals to red. You have to be careful that the cure isn't worse than what it is curing.
It is also worth bearing in mind that there are a plethora of other things that could stop all trains in the Gloucester Road Junction area, some far more likely to happen. In relay interlockings of that era, there are some key relays and circuits that if they fail, will bring the whole interlocking to a halt. For example, I seem to recall a button fault on the East Croydon panel ring that stopped all routes being set for a couple of hours.