Yes, London lounge is very very crowded most times. I still did not get the idea why AC and SAS combined their respective lounges into one.

However, I have also used SG's lounge. In terms of quality, I am not so sure SG's lounge (which is also open to * golder) is better than AC's London lounge.

But certainly SG only has 3-4 flights per day, so in terms of traffic, once you pass the noon rush hour, SG lounge is extremely quiet. Once I was the only passenger for 3-4 hours. Also it is located in a less traffic area.