I'm in a pretty similar boat to you Sebbelcher. Plat Pro on AA, but increasingly flying on BA metal and thus would prefer to switch over to BA's program, given that BA loyalty pax have a greater chance of an upgrade (although I've had a few given my OWE status, you will always be a lower priority than BA's own). Also, being with BA's program gives you the ability to use avios to upgrade one class, which you can't do as a OW partner pax. I'm currently based in Florida (but have a rotational job in Nigeria flights to and from via my native UK), but will be repatriating to UK in a year or so's time (after 20 years in US). I'm also unlikely to requal for Plat Pro come the end of Feb (will drop to Plat).

As some of the others have said, when booking avios award tix (I also have a sizeable stash of Avios due to lot of flying paid QR J in previous years at a different job), to get your AA status benefits you just need to put in your AA FF# on your BA booking. Note this can sometimes be a bit tricky as I have found if you book as yourself via your BA account it hardcodes in your BA# as unchangeable (but can be gotten around by going to another OW carrier site such RJ or Finnair and changing there, or at least clearing the FF# then going back to BA's 'Manage a booking'). To avoid that hassle, I've found it is much better to create another passenger (with your own name) and then use the drop down to select yourself - then later, when you get the booking, on 'manage a booking' you can input your AA#. Similarly, if you were to start collecting BA avios/tierpoints on paid bookings, you can still get in the lounges by way of showing your AA card at the airport - but I can't see a good workaround whereby you get to pick your seats free and ahead of time (via inputting your AA status number), then changing your FF# back to BA. I guess it may be possible at a BA check in desk just prior to boarding - I may ask later this month when I'm bouncing back through the First Wing at Heathrow T5.

For your last question on generating AA LP in the UK (via credit card) - as you say I don't think it is doable currently, I know there are conversions available for AA miles with Marriott Bonvoy etc, but I guess they will just translate over as miles not LPs. For me (being a dual UK/US citizen) when I move back to the UK (and if I decide its just too much of a hassle to switch over to the BA program) I will just keep my Aadvantage US credit card going - there is no foreign usage charge on it.

I really do wish it was a bit easier within OneWorld to effectively move from one carriers program to another - but as you said, they don't seem to allow status matches for that purpose and the only real way round it is to give up seat picking, baggage and boarding priority benefits for that year (but not lounge access) till at least mid-tier status is achieved.