Obviously you're new to this

There's no set route that the aircraft will be from. You can search BA11 on FlightRadar24 and once you get near to the date, it will tell you the aircraft that's scheduled to run the route. You can then track that particular aircraft on the day to see whether it's running on time for its trip to LHR.

The aircraft are always subject to change though, even at the last minute if there's delays etc so you'd have keep an eye on it constantly. And there's nothing you can do with the information because BA will still expect you to turn up at the airport on time even if the incoming flight is late (in case they switch you to another aircraft).

From the last week's activities it's been the LAX and SFO aircraft running BA268 and BA286, which basically makes sense with the early afternoon arrivals.