You also get full miles if you use the Finnair (AY) codeshare on the same route, and Finnair often has better availability at a reasonable price than the AA codeshare.

In fact, I fly that route a fair amount, and have never gotten the AA codeshare LHR-ARN. Occasionally I have had it in the return direction. Most often I bite the bullet and book the BA flight number, accepting the lower mileage because the only alternative is to pay more for that flight than I do for the transatlantic portion.

Too bad the ORD-ARN flight is long-gone.