I'm booking a flight on BA (SEA-LHR-SEA) and I have OneWorld Ruby status (via Alaska Air). With my status BA says I can pick seats 7 days in advance:
(britishairways[dot]com/content/information/seating/reserving-your-seat)
Bronze/oneworld Ruby
Free seat selection for you and everyone on your booking from seven days before the time of departure on British Airways flights, excluding group rate bookings and exit row seats. You must be travelling on the booking for the benefit to apply.
Exit row seats can be selected for free from 24 hours before departure.
Does this apply to the Economy Basic ticket? I don't see any qualifications indicating it doesn't, but I just want to double check.