The timetable can work.
One driver drives Peterborough to Horsham, has a break, and drives back. Easy.
Something goes wrong? Peterborough driver takes train to Kings Cross, then empty to Hornsey to clear the platform, has their break, takes train back into Kings Cross, and picks up return path.
Easy.
That's how it should work.
But what's happening is that the Peterborough driver drives to Finsbury Park, (and then is booked to work another train North from Finsbury Park and hasn't got enough time to go into Kings Cross should something go wrong such as no Horsham driver to relieve him).
A Horsham Driver takes over to drive to St Pancras through Canal Tunnels (hastily learnt with weeks or days to go!), then gets route conducted through the Core to Blackfriars. And then route conducted again Blackfriars via London Bridge high level (only signs low level!) to East Croydon, before continuing by themselves to Horsham.
And that's assuming they are one of the ones who were trained on Canal tunnels.
Thus it needs four drivers for that train, and any delay in the other direction making one of them late, or one of the jobs being uncovered (there's a lot less spare coverage than there would've been had route knowledge not been a problem), and it all degenerates into chaos very quickly.