Originally Posted by Gardyloo
On discounted coach you can't even join the BAEC, so that option's out.
AA will indeed give you 25% miles on discount Y. Asiamiles (CX) will give you 50% miles in most discount Y buckets (down to R) so that might be a better choice; Asiamiles are slower-earning than some programs but have better award features on some itineraries than some other plans. If your other travel takes you to Asia that might be worth looking into.
I thought AsiaMiles expire after 3 years no matter what (can be renewed for a fee). If I'm not mistaken, you should take that into account.
Also, you could earn AA miles on the BA flight if you route through Canada. Just a thought.