Talked to the No1 lounge staff this morning, at this point there is no intent to have fights / gates called in the lounge, including BA pre boarding (this is a No1 rule they said).
The point they are missing is that JER is designed differently to most if not all other airports where No. 1 operate, in that the gate number only becomes available at the point when the gate staff are genuinely ready to process passengers. Hence if you want to take advantage of BA priority boarding or EasyJet speedy boarding, you have to head to the gate once it's announced. It's very different to LGW or LHR where if you head to the gate once it appears on the boards, it's likely boarding won't have started when you get there.
Additionally the priority announcement at JER is verbal-only. If you're in the No. 1 lounge and don't hear it you can only go to the gate once it appears on the boards (at which point all passengers can obviously see it). As a result if you're in the lounge you're essentially certain to miss priority boarding.
I can completely see why they don't make announcements at LGW for instance (it would be insufferable) - but at JER there are far fewer flights and the airport operates differently.
If anyone has a senior-ish contact at No. 1 feel free to PM me as I'd like to drop them a line. It's a beautiful lounge but I've been caught out twice by this point already and it's annoying especially when you need overhead space for carry-on.