Can’t comment about breakfast specifically as I don’t eat breakfast at any ARN PP lounges.

But given:

1. the location of the ARN lounges are after security but on the Schengen side of the airport (i.e., before exit passport control); and

2. the only ARN lounges where I find acceptable meal food for me are the Pearl lounge at one end of the airport and the Amex Centurion lounge at the other end of the airport,

I do breakfast at the hotels whenever possible or a grab-and-go type thing in the mornings.

Also, the Reisen breakfast is superior to what the SAS lounges have and also superior to what the Priority Pass lounges seem to have. Pearl is a Priority Pass lounge too, but they will cut off PP access when they think they are full of premium passengers for contracted airlines or when their online link with Priority Pass and other lounge membership programs goes down. The good thing for you would be that if BA is covering the lounge for you, then you’ll get in to the Pearl lounge. [After security, at T2, it involves going upstairs the same way as those going to the O’Leary sports bar/restaurant or the sealed off inside smoking area near it.]

The Reisen breakfast is free for Hyatt Globalists, but for most other guests it comes at a cost unless the rate plan includes breakfast.