Apparently it's fixed now, and priority is being given to inbound flights - i.e. landing them all.
Call me a cynic, but it was only a few minutes before our (ex-LHR) doors being shut, the bridge retracted, and "by the way" we've got an hour's delay because...
Have we moved an inch? No. Do we count as an "on time" departure - you bet
, even though we're unlikely to start moving until an hour late.
(in fairness, I guess it's not BA's fault...)
A flight is only classed as departed when it pushes back off stand.