You earn AA miles on AA codeshares to the UK even if they are on BA metal. You also earn BA miles on BA codeshares to the UK on AA metal. AA/BA are prohibited from codesharing at LHR and LGW for flight to/from US. BA does codeshare on AA flights at MAN (ORD-MAN, BOS-MAN and MIA-MAN have all had BA codeshares), and AA codeshares on the BA flight MAN-JFK. BA has chosen not to codeshare on other AA services to the UK (e.g. ORD-GLA was not a codeshare, when it operated), and BA does not want to operate any trans-Atlantic service except from London (the MAN-JFK flight is a political anomaly and BA keeps trying to kill it).

As for CX being a good plan, yes and no. It is a much more complicated plan than AAdvantage with generally worse earn/burn ratio. But it has lots of special features (some good, some bad). Diamond is much harder to earn than EXP (really equivalent to double EXP), thus much rarer and with more CX benefits (but not more Oneworld benefits). Those living in HKG and able to make Diamond are much better off with CX Diamond than with any other OW status. And CX often sweetens the pot with unofficial Diamond benefts, some of which are quite valuable (but not guaranteed).

One example of the complexity of CX plan: awards US-Europe vary in cost (based on mileage) while on AA it is the same cost (anywhere in continental US to anywhere in continental Europe). So the comparison of CX to AA varies by route for award redemption (and there is a big percentage difference). The same is true for the rest of the plan. Which is best depends on how you earn and burn, it has to be an individual analysis if you want to optimize the miles.