The daily 49/48 rotation just switched to 747's in the last week, and they so far have been running 86J's Super Hi J's except one day. Tonight's (Sunday) flight started as and 86J, then dropped to a 52J earlier in the week, and switched back to a 86J yesterday, as I was going to be on it.

The SEA route the last few years has been 4 Class new cabin 777-200ER's during winter on both rotations. For the Summer season (since 2014) they have switched the daily rotation with a 777-300ER. I took a break from flying out there last year, so didn't pay attention on what was used on the route during the summer of 2016. It is a very business heavy route for BA, so if they are going to run 747's again daily, I would expect them to be the Super Hi-J's.
And since Virgin the last week as taken over the route from Delta, flying 789's, when Delta were running old 767's, for customers it is an upgrade. BA the last few years haven't run aircraft with the old shoddy cabin offerings on that route, and if they do, I am sure they will gift Virgin a wedge of business travel.