Last time I checked the BA timetable, they operate 6 times on a Sunday to LHR and not the 5. The 767 operated BA1325 which is the 3rd flight of the day, at 1355. Listing the flight before and the one after from flightradar departures:
BA1327 scheduled 1215 departed 1206
BA1325 scheduled 1355 departed 1355
BA1333 scheduled 1445 departed 1442
As for it being a 767, perhaps Titan didn't have any other spare equipment and I guess BA had none either as they subbed it out. Does it really matter if it's "apparently" too big?