Where I work, it has been known for ECS runs to be converted to unadvertised expresses if there are so many as just a couple of passengers who have been displaced through no fault of their own, where the last public service has otherwise departed, but there are still crews available (etc.). If it's been in the system a while, that's a bit odd, though.
Obviously the Virgin service in question would have arrived at Birmingham at a point at which you would expect early morning travellers to London to begin arriving. Perhaps it was somebody's bright idea that an ECS from further North could be converted to a through passenger working in case anybody was stranded overnight? Perhaps the Sleeper was unable to accommodate some passengers from Carlisle and an arrangement was made? So many possibilities!
As for a charter service, that would make for a very long day out for somebody, and presumably at pretty great expense for VTWC, who would have to no doubt organise quite a number of extra movements and crew. Obviously VTWC also don't have access to Sleeper stock, so an overnight journey to London would surely not be for anybody particularly prestigious or who cared about sleep. Could perhaps have been to transport a team from some unknown company or organisation carrying out a movement of engineers/light equipment overnight for logistical reasons.