Did the return from Oz a couple of years ago, anywhere in the Far East is about 11-13 hrs from Heathrow depending which airline you use and which route they are taking, but most of the time you will be either asleep or eating, so just enjoy it.
Sometimes a good tailwind can shorten the journey time, bad weather can lengthen the route... it's all very flexible.
Unless you have a taxi booked at Bangkok (and his clock is ticking) just enjoy the flight and when you get there make the most of your time doing the touristy trips.
Do the canal/river buffet lunch cruise , the Zoo/culture show and the Golden Palace trips.... get out into the real villages and countryside. Try the public transport monorail and see the real life Bangkok city.
One word of warning.... tell the " I am a policeman on a day off" blokes to shove off... they are con artists who will do owt to make a dollar. The tourist board has a phome line specially for reporting these guys.
Above all, enjoy your trip.
