I used
DCL transportation and I want to say the bus I was on left about 9:30/10:00 and we got to Heathrow about 12:30 or so. I had a 3:30 flight, so I had time to check-in without rushing. At one point, Delta actually changed my flight to 12:00 and I had to call and tell them DCL said it had to be after 3pm. Of course, if you book an earlier flight you can hand carry your luggage off the ship, but I wouldn't make the flight too early.