It’s an interesting question what diversionary knowledge Horsham-Peterborough drivers have, given that GTR has tended to silo drivers onto particular routes. I don’t believe it is now possible for them to simply divert down the Cambridge branch, for example.
Back in the days before 2018 Peterborough drivers had booked work to/from Cambridge. I don't know if they retain that now.
For resilience purposes, the key requirement is for the Horsham-Peterborough drivers to be able to get to Letchworth, where trains can reverse on the A/D lines or be dumped in the carriage sidings.
Peterborough drivers do still sign Hitchin to Cambridge, and have booked work.
Horsham drivers on the other hand, do not. Not even the shunt signal around the corner on the old Cambridge line at Hitchin, so cannot turn a train around there.
They also don't sign Hornsey Depot, or Welwyn sidings (although you can only fit 8 car trains in there).
Hitchin drivers sign to Peterborough and Cambridge, but don't sign 700s.
Kings Cross drivers sign Peterborough and Cambridge and from this year some now sign 700s, with all of them planned to do so.
Welwyn Garden City, Cambridge (both links), Hornsey Outer link and Brighton (Cambridge link) sign 700s and Hitchin to Cambridge but not Hitchin to Peterborough.
Brighton do not sign Welwyn sidings or Hornsey Depot either.
Before 2018, all GN depots signed all traction, and all routes except Cambridge and King's Lynn didn't sign Hitchin to Peterborough and they were the only ones to sign Cambridge to Kings Lynn. There were some sidings and diversionary turn around points like Cannonbury Tunnel which were added or missing between them, but there was a lot commonality.