Thanks all, good suggestions. I ended up booking separate back up award reservations with AA miles, which as an EXP are fully refundable. So as long as I know of any cancellations 2 hours ahead of the flight, and BA hasn't re-accommodated me, that would give me enough time to switch terminals and go with plan B.

The only risk is a last moment mechanical cancellation and I am on an AA ticket and I if I fail to get rebooked quickly on the morning flight, but what are the chances of that, lol...I'll better sit in the front of the plane as much as possible and have the EXP phone line handy. Again, small risk though. Even if mechanical happens they should be able to sub the aircraft, it is LHR after all, right?