Could it not be a combination of the two, with ATC causing the bigger problem?
I.E. Crew member has a situation come up suddenly which prevents them from being able to fly. That then leaves the aircraft without enough crew and is an unforeseen event which is outside their control.
Then ATC issues developed, either at BRS (were there other delayed flights in or out during the time?) or en-route, or at your destination, during the time spent waiting for the replacement crew to come from LGW, which pushed your departure back further than expected.