Posted delays are usually observed, in order to avoid ticking off too many passengers, but it's always possible that a delayed flight ends up leaving earlier than expected. If you arrive at the gate any time after 10 minutes prior to the scheduled departure, you're technically at the mercy of the GA (although I've generally found them to be pretty reasonable, especially for int'l departures).

IIRC, the curfew is not an absolute cutoff at 6:00am. There are a small number of overnight flights into LHR, but they're very limited. I would assume that in order to schedule a 5:55am arrival, United managed to snag one of these exceptions.