I've done it twice - once via the bus (which worked well, but we had plenty of time) and once via private transfer (this past March), which worked out better - more comfortable and nice and clean / tidy / bottled water, help with luggage etc. I used the same people who had organised all of my transfers for this last transfer, given the late hour. Traffic was quite snarled, but we made it. If you're travelling in a premium cabin on BA, look out for the uniformed airport staff outside the terminal with radios and lists. They'll get you through the door to the airport quickly and right into the hands of special services at the premium check-in desks.