Agree with the general advice above. You typically will only get in to a higher cabin if you paid for it at the time of booking, or exchange Avios, cash or vouchers (for higher status members) to get in. Free upgrades are not part of BAEC in the same way they are in North American frequent flyer programs.

However, as with all rules, there are exceptions. The most common of which is the operational upgrade (OpUp) when BA have oversold a cabin and need to move people forward to accommodate other passengers.

Very, very occasionally, you might get a complimentary upgrade. Example: travelling with a colleague earlier this year from the US back home, she Silver, I’m gold, and 24 hours before departure, she’s upgraded out of WT+ and in to CW. Her boarding pass printed at the airport reads: “enjoy your upgrade” … this was early January and both WT+ and CW were around half full. So it was not done for operational reasons. She was a newly minted Silver and hadn’t flown CW before. I’m guessing it was done for commercial reasons to encourage future purchasing decisions.

So it can happen, but for expectation management it’s best to work on the assumption that it won’t, and be pleasantly surprised and grateful when it randomly happens.