This isn't unusual, and the usual victims are the late domestics if they get a bad run of creeping delays thanks to weather. If T5 gets several aircraft after 23:00 hrs the message you get is fairly standard, and what you did was sensible: wait the usual time, maybe add 5 minutes, then leave for home. I think there may be as few as 4 handlers on call, which would be need 20 minutes per shorthaul aircraft and twice that for longhauls. BA's handling of overtime for ground staff is, I have been led to believe, open to improvement. Then as the message indicated use the online baggage tools to register an entry in WorldTracer. If you are London based you would normally get the bag about 2 days later via a courier company. BA no longer requires people to submit claims at the airport.

ba.com/baggage

LGW also does this too, but they are staffed better for the late AGP service (etc) so it takes a lot for this to happen there, it sometimes happens if there are labour shortages, many of the baggage handlers are from Eastern Europe. I'm not aware of it happening at LCY or STN.