I had the same lucky experience both ways, BOS - LHR. I booked World Traveller Plus BOS - LHR and LHR - BOS, and on both legs, I got a surprise, unasked for upgrade to Club World. It was an amazing treat! I am AA Platinum (so OneWorld Sapphire) and was using my AAdvantage number rather than my no-status BA Executive Club. Right at T-24hours, I went to check in on the app, and my seat had been changed to Club World. The experience was great on all fronts.