Which angle of twilight to use for prayer time calculations?

I live on an island on the south coast of england. Recently a city that is close to us on the mainland has changed its prayer times so that Isha has become later and fajr has become earlier. I believe this is due to using 18 degrees as the twilight setting. However, we have a timetable that uses 15 degrees as the twilight value hence fajr is later and isha is earlier. Another neighbouring city also uses 15 degrees. Which timetable is correct and which one should we use?