In this case, you should go to the S7 lounge for the first 6 hours of your layover and then move to the BA lounge (it opens at T-3 prior to BA/IB flights). It's 2 minutes walk between the two.

S7 lounge has a nice relaxation area on the second floor, with dimmed lighting and nice aircraft style reclining chairs. They are also installing similar armchairs downstairs, so you can lounge while enjoying the view of the apron.

On the way back, the S7 lounge is your only option.