OK - you will only have 2 days free in London before the rest of your group arrives. Or maybe 2.5 at very most. And at least some of you will be jet lagged and or exhausted. That could mean very tired and sleeping a lot, or it could mean totally screwed up body clocks where you are wired and don't get to sleep til 3 or 4 in the morning. So you have to make allowances. And no matter how everyone is coping, you will NOT see all the major sites in London - not even a tiny fraction, let alone have time for a day trip.