Sorry not exactly answering your question:

The US/UK checkin line, which is separate than others inside the M&S Elite checkin area, will close the night before around ~1h before the last US/UK flight and will open 3 hours before the earliest US/UK flight in the morning. However, I had also been told that the earliest is 3 hours before, but as soon as the area opened for another flight earlier than mine, I could go through in the morning. Never tried it the night before though.

Hope someone else with more insight will share their experience, but in any case if you're going to do it the night before, make sure you arrive there when the area would be open.