Bangkok to London is about 9,578 km and usually takes around 13h in the air. The track it follows runs over the Eastern European plains, Central Europe, and the North Sea.
Over the North Sea, the Eastern European plains, and Central Europe the air is usually calm and settled. This track threads jet-stream latitudes, so in winter you may feel a few minutes of light, choppy air high up; summer usually rides calmer.
A flight this long spends hours at a high, smooth cruise. Even when one stretch is bumpy it is a small slice of the whole trip, and the aircraft treats it as routine.