No problem with BA booking this flight - it's a proper BA 'flight' bookable on BA.com

Re the second point - I think my search for brevity confused matters! I meant get a flight from MAN-DUB ( probably an economy direct on Aer Lingus or equivalent BA codeshare on a separate ticket or even ... gulp .... Ryanair) THEN pick up the American flights to PHL, CLT, ORD etc. May save a bit of tax and preclear but obvs not on the same booking so not protected.