I saw many pictures of noctilucent clouds in the UK. Is it a phenomenon only visible in the UK or we can see it somewhere else?
Noctilucent clouds form during summer at high latitudes, such as between 50 and 70°N. They are noctilucent because at their high elevation (80 km), the sun is still shining, although the sun is below the horizon at ground level. Due to this high elevation, you can see them from several 100s of km south of their actual location. This means the ideal viewing location is at around 55–60°N; the night there gets dark enough throughout the summer, while the noctilucent clouds a bit further north are still sunlit.
If you go too far north, you will not see them very well, because the night will be too bright. I lived for 5 years at 68°N, and although they could easily be detected by ground-based lidar, I never saw any myself.