I was in the lounge then too and indeed I had been due to be on the BA16. Luckily I had been put on the BA10!
http://www.flyertalk.com/forum/showthread.php?t=540631
I heard the comp offer too and thought it was fairly paltry. Still, I expect they start low and if they get the volunteers they need, then there's a winner. No point starting high if you only need one or two volunteers and having a queue 10 deep at the desk. I expect if they hadn't got anyone to go for it, then they would have upped the offer. I didn't hear them make an improved offer, so assume someone must have gone for it. You wouldn't catch me, or many other FT'ers I expect, taking that kind of downgrade comp though!
I discussed it with my seatmate on the BA10 who'd also heard the offer and he thought it was on top of the fare difference refund, although I'm sure they didn't make that clear at the time. But I would think that must have been the case though, although interesting to note which Y fare they would have refunded the difference for, one would assume the full Y fare I guess.
Did you try any of the Krug? Disappointing to only get half a glass. There didn't seem to be any other Champagne offer in the F lounge, which I found disappointing, only some sparkling wine option which didn't taste that good.