I went through recently on a very delayed flight from LHR. My plan had been to spend the day in Olso city before my return to LHR in the early evening. That prospect vanished as I ended up with only just over an hour to go back.
So I went to the passport desk where the officer listened to my story. He sent me to the above-mentioned door where I rang the bell and waited. After about 10 mins I was taken through to the small security section and processed into the non-Schengen area. I asked a friendly customs officer if I could get to the lounge and he explained where it was and directed me to the closed exists from the passport desk. He told me to knock and wait. One of the doors duly opened, I repeated my story and was allowed into the Schengen area, and could thus access the lounge.

I reported it here.

https://www.flyertalk.com/forum/brit...tinations.html