For what it's worth, OP, you could fly AA to LHR via LAX in reverse herringbone J and only lose out on the SJC-LAX miles if crediting to AS. But any onward connections, besides incurring the manual boarding pass submission hassle if you've booked them as AA codeshares, will most likely involve a Heathrow terminal transfer, and... ugh.
I've had MVP for the last two years as a PDX-based leisure flyer, and my status is good through the end of 2018. AS has treated me very well, but my upgrades won't come quite so easily here, and with the partnership losses it's just getting harder for me to maintain even base status. I've been flying 50-60k the last three years, but between reduced earning rates on every partner but QF's discount coach fares and travel to places where the AS partner network just doesn't go--TBS, anyone?--or doesn't at reasonable prices for my weird itineraries, I've only been hitting MVP. The loss of DL really hurt, but I suppose the writing was on the wall for me as soon as AA devalued their program and AS earning rates dropped from 100% to follow suit. So far I'm set to earn a measly 6268 AS EQMs next year, and while more bookings will definitely come, I have no chance of hitting 25k.
So, now that I'm in SEA I'm acting as more of a free agent for 2018. I have a few AS bookings where the price was right and I have a shot at an upgrade, but I already have some reservations on B6, AA, and UA. I'm also burning through AS miles while the charts are still good--one AA South America open-jaw in Y a one-way in F/J for next year, plus tentatively US-TYO-SIN on JL and HKG-Europe on CX for 2019. But long-term I'm probably just going shoot for DL SM. My RDM earnings will be terrible, but I'll at least have an easy time earning MQMs at 100% on coach fares (as long as I avoid Basic Economy) from DL and their closest partners, and maybe a shot at GM in years when I'm feeling especially spendy and/or loyal. But I've already done enough fare research for future trips to know DL isn't always the cheapest internationally from SEA, and I'd much rather fly, say, KE or OZ in Y anyway. My only big question for now is if I'll waste my once-in-a-lifetime DL status match to get some benefits in late 2018 or just make do until the next year and get it organically.