Issue at KEF as has been mentioned in another thread recently is the lack of non-schengen airbridges. When the new non-schengen area was built in the early 2000s it wasn't anticipated that Icelandair or Wow (which didn't exist then) would need any more gates than were built. Forward 15 years and Iceland is even more popular amongst non schengen visitors.

I would imagine the empty gates you are seeing are scehngen gates and therefore can't be used by BA.