Logged into my executive club on the PC. Manage my Booking>Upgrade>Your upgrade options. On that page there was a link in red type in the area with a red oval in the screenshot. I tried to upload a screenshot, but I can't (tried two different browsers), sorry.

I don't know why I got the offer, maybe because I've used the AA alternatives quite a lot and I'm sure BA would like that to change. A welcome back offer? I'm also sure BA hopes I'll be so impressed that I'll upgrade the return leg at full whack... which is £1,454 when I click through. I'll keep checking in to see if that changes. In any case, it seemed pretty reasonable to me. Looking forward to it.