Welcome to Flyertalk and welcome to the BA forum frankleggett.

Past performance isn't necessarily a guide to future outcomes, but I'm sure you are aware of the pressures on the aviation sector at the moment. LHR isn't as badly hit as some places, but that pressure is part of the answer. IAD has also had some challenges in this space. You said BA239, which is the Boston service, did you mean BA293? That's an A380 service so there is a specific set of issues there: firstly they are being run flat out at the moment, they are having minor technical problems, and with so many people on board things like baggage and passenger reconciliation just take longer. Much longer, as you have seen.

BA will email you if there is a significant delay, that email has two core forms. One form says "there's a delay but please check-in as normal since we will try to reduce the delay if we can". The other is "there's a delay, no avoiding it, so you can check-in late". You can more sensibly look at this the day before departure, the day of departure, I don't see much point thinking about it now.