Seems to be a lottery. Been Gold for 6 months and Silver for most of the 14 years prior to that. Been upgraded on 5 occasions, of which 3 were in the past 3 weeks, all coming back from TLV on flights that were overly full.
I don't know BA's rules for 'operational upgrades' but I suspect it goes along the line of flight capacity vs. value of customer to the airline. Why bump someone up to Club on an empty flight? But you can see the logic of upgrading someone if economy is full and you can get another paying, warm body on the flight by making some lucky Gold cardholder's day. Suspect the customer's attitude has a lot to do with it too.
Last week I had 27B on a 767 which is a bulkhead seat and you can prop your feet on the attendant's seat during the flight, so not the worst seat in the house. Didn't even ask for a seat change at check-in. But they bumped me up, doubtless to sell the seat to someone else, and I got an even nicer ride home.