I don't think it's reasonable to suggest passengers from Derby to London go all the way north to Doncaster; this was clearly a mistake.
Doncaster is a bit out the way, but as
@DarloRich has said, sending everyone via Newark would just crowd that area too.
Different stations will advise different things depending on their location.
Leicester/Market Harborough may send everyone via the Nottingham/Newark route.
XC may deny access to the Stansted train because the TM has told XC Control how busy it is.
Another good one is the Rugby - Birmingham route.
Rugby, Coventry and Birmingham International
should (or used to) send their passengers on different routes so as not to overcrowd one particular route, even if it seems most logical / shorter distance to go that way.
When I looked at the staff briefings for VTWC PIDD routes, it was all very well thought out.