(Re. Post 180 in particular...)
The problem is more that it is well-known that the drivers don’t sign the new routes, but they are rostered for routes for which a route conductor (known by TL as “pilot drivers”) should be available. This has its pitfalls.
When you pick up your diagram/schedule card for the day, you are meant to check it complies with your route and traction knowledge - or ensure somebody can conduct/pilot you by accompanying you. This system applies across the country to both drivers and (where applicable) guards.
If you are a GN driver and somebody assures you that you will pick up your pilot at Station X (say Finsbury Park) and they will show you the route from there, then off you go, driving your train to meet them. Then when the service arrives at the appropriate point, you wait for your pilot. If all goes well, they’ll board the train, confirm the necessary details, and off the service goes.
Unfortunately every conceivable driver who signs the new routes via St Pancras / London Bridge is route conducting, and the resources are currently deployed with very little slack/flexibility because it’s a case of “all hands to the pumps and we might just cope”. Therefore the slightest thing which goes wrong will displace the crew who are competent to take trains on new routes through the core. This is basically what happened a few times yesterday afternoon, building up into something of a chaotic backlog.
Running out of pilot drivers is definitely something which is an operational difficulty, though that is not necessarily the best explanation for the public domain. Sadly I am not sure the CIS systems on stations can actually process and clearly explain the exact reasons, as they may not be present in the pre-recorded messages - and the announcements against trains on the likes of the NRE website generally also use preset text, which is sometimes not that easy to modify.