Dubai to London is about 5,498 km and usually takes around 8h in the air. The track it follows runs over the Middle East, the Anatolian plateau, the Black Sea, the Balkans, the Eastern European plains, Central Europe, Western Europe, and the North Sea.
Air lifting over the Anatolian plateau can raise brief, light ripples at cruise; crews know these spots well and the movement stays gentle. Over the Black Sea, the North Sea, the Balkans, the Eastern European plains, Central Europe, and Western Europe the air is usually calm and settled. Above the Middle East daytime heat can stir shallow thermals lower down, but the air smooths out at cruise. 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.