Manchester to Istanbul is about 2,665 km and usually takes around 4h in the air. The track it follows runs over the British Isles, the North Sea, Central Europe, and the Balkans.
Over the North Sea, Central Europe, and the Balkans 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.