All of us would be delighted to do that -- but CX doesn't upgrade using AAdvantage. You can upgrade for Asiamiles but in practice need CX DM status to have much chance for the upgrade to clear.

Be aware that OW awards prohibit routing and airline changes -- only date changes are allowed. So you cannot switch from BA to CX on HKG-LHR on that OW award (even if award inventory is available). You would have to refund the award and issue a new award (hardly practical). I think the die is cast and your routing is locked in, given your constraints. Presuming you are in SAN, both of you could have flown BA J (to any of a dozen cities) and found it as good or better than AA F in terms of comfort. If you happened to choose one of the BA J+ routes (maybe they are all upgraded by your flight date), it would be superior to current AA F in terms of quality of food, wine and bed. Ironic isn't it that some J is superior to some F? You mention wanting to maximize the F experience, and my critique is that it is closer to being minimized than maximized (even QF is superior to BA on HKG-LHR so it really is the worst choice on that route, as is your LHR-USA leg).