Thank you, everyone, for your responses. I did call again this morning when the US line opened and because the F seat was still available. But ran into the same problem. This time a solution, which is to unmarry the segments. Time not on my side as this unmarriage (or is it a conscious uncoupling) must be done by revenue management and they close at 5pm GMT, 10am here out west which was just after I called in. Daresay the F seat will have gone by the time those guys return to work, but the again very helpful, and this time UK agent, was very supportive even suggesting that because we have tried to do this in good faith, the revenue gal she knows "well" might release a new avios F seat. Well, we'll see. And if that happens, I'll certainly be back to update.

Anyone looking for a F seat LAX-LHR? This is on the slightly less popular and earlier 282 on Maunday Thursday.