Just to add my 2 cents. We spent that New Year's Eve in London while enroute to LCA. We had an early am flight and were not interested in partying. The activities seemed to center around the central areas of London. We wandered around the Covent Garden area in the afternoon. It was fairly crowded and was getting more so by the minute. We left the area around 5 pm and traveled out to our hotel by LHR. We had a very noisy night as the revelers seem to expect everyone to join the party. After a couple of hours of interrupted sleep, we escaped to the airport for a cup of coffee and to await the opening of the Terraces lounge. We were then able to relax and enjoy the peace and quiet of the lounge in the early hours.

I'm sure you'll have no trouble finding some kind of party if that's what you're looking for. Have a nice time.