Istanbul to Manchester is about 2,729 km and usually takes around 4h in the air. The track it follows runs over the Balkans, Central Europe, the North Sea, and the British Isles.
Over the North Sea, the Balkans, and Central Europe the air is usually calm and settled. Nearer the British Isles the weather turns more Atlantic and the ride can be livelier for short spells, still firmly within the routine. 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.
For most of a flight this length you are at a steady cruise above the weather, where the ride is smooth; the livelier moments, if any, are short.