I have found that the award taxes aren't bad at all if on your outbound leg you transit at LHR from AA to intra-European BA because the value of those intra-european BA segments is so low that the taxes are not significant. Returning thru LHR is a different story though ...

My strategy lately has been to transit via LHR on the outbound, use BA to get to the exact jump off point I want and then plan travel so that I can return on AA metal direct to US out of one of their gateways (although it seems like every European country has some form of departure tax, just not as high as UK).