I was at ARN a few hours ago and checked up on this. It turns out that there's a bus gate called F1, which is located to the right of the passport control booths where QR passengers enter into Schengen. You can press the button there, and a bus will take you to gate 70 in T2, which is in the non-Schengen part of T2, and it's just a quick elevator ride to the BA gates.

But note:

1) You will need to wait at gate F1 for a bus and a security officer, because arriving from Qatar, you'll need to go through security before boarding the bus. I have no idea how long this will take you.

2) The business lounge in T2 is inside of Schengen, so you'll miss out on that lounge if you take the bus from F1.