A few months ago, I booked a last minute LHR-JFK, on the last flight departing at ~19:00. Since I booked only the evening before, I was surprised that I still got a bulkhead WT seat, so I was happy with that.

At flight connections, I got the magic beep, I was upgraded to WT+. I declined the upgrade, as it was a centre seat, and I find a bulkhead aisle seat in WT to be superior than a middle WT+ seat, as I like not having someone recline into my space, as well as being able to get up at my pleasure. ALAS, some other Gold or Silver got a upgrade.

Then at the gate, I got another beed, at first I was hoping that they didnt "force" me to upgrade to WT+ and give away my bulkhead seat, but alas, I was double op-uped to CW!

I believe that the last flight of the day to JFK has the best chances of overselling, and as there is no other flights later BA could accommodate an oversell, it is cheaper to upgrade than to pay for a hotel!