so yes 2 chimes is about to takeoff usually on runway entry at LHR. but for places where you have a significant back track (e.g. INV, CFU) you may get that later just as they are about to turn at the end to line up rather then on runway entry.

1 chime shortly after takeoff, and again about 1-2 minutes before landing, is once gear is up and gear is down respectively.

it is just supposed to be 1 chime usually somewhere between 6,,000-10,000ft for seat belts off, basically once it is deemed safe to do so. usually cabin crew will be up well before that, especially for the shorter routes. occasionally you do get 2 chimes for "releasing the crew" which I can only assume is because there is a reason they should remain seated until a positive signal by the flight crew - expected turbulence maybe? otherwise default is crew decide when to release themselves and don't get a chime to do that.