Probably not at this point.

You're in the shoulder season, so at the very least travel after 15 September for lower fares.

As you get closer to the date, BA will restrict inventory so it will likely be fuller fare tickets to screw over business travellers.

Book now and at least be assured you've got what you want.

Also, bear in mind that SEA flights go into T4, whereas if you want to fly on BA only then their DUB flights depart Gatwick. You can go via LHR, but it will be on Aer Lingus (the Green RyanAir)as a code share and they charge for everything now: bags, food, drink, etc.