The Brit seems to be the only prisoner who doesn't want out, is this consistent with a Nemesis- infection?
Maybe it was the other way round: only part of a being/person was entombed there in the first place. Like a spiritual anchor (yes, Horcruxes inspired this WAG). Demonreach was constructed in multiple times, maybe in order to keep the prisoners out of time and thus reality completely. A spiritual anchor there could enable a time-magic-savvy wizard for example to flee into the prison in a crisis, instead of being killed or sent to a place they don't want to go.
In short: I think the Brit could be Chandler, who could very well have been the Warden of Demonreach we have met, who is more or less modern upper-class British, who understands time-magic, who disappeared mysteriously without a trace instead of getting killed and turned by the Black Court ... and who probably asks Harry next time to be released now.