I'm not sure that's massively different from having to comply with EASA restrictions on crew resting time, which also causes reschedules on a daily basis. However, generally speaking the only delay that matters is the one on arrival, so they can advertise a departure delay for as little or as much as they like, but it's doors open in King Khaled International that is relevant here.

Unfortunately the dispatch information is no longer available - it only shows for a day or two - which would give more useful information, but the limited information that does show suggests a touch down in RUH at 01:21, against the advertised 22:35 hrs, so given they are fairly swift to the gate in RUH I suspect you squeaked in under 3 hours anyway, unless you have your own timings to supersede that. If there was a 10 minute ATC delay then you would be certainly out of luck there. The take off time is showing as 16:38. It also shows as gate A10 at T5, so it doesn't look like BA's finest effort all round, other than that they did inform people in advance such that they could arrive late at LHR.