Allah ﷻ has prescribed five Salah in a day and designated each a window of time as mentioned in the Qur’an:
إن الصلوة كانت على المؤمنين كتاباً موقوتاً - Surely, Salāh is an obligation on the believers that is tied up with time. (Surah Nisaa, Ayah 103 – Translated by Mf. Taqi Usmani)
The Prophet ﷺ has elaborated on the times of the five Salah. These times are based on dawn, sunrise, midday, 1-2 lengths shadow, sunset and dusk.
Fundamentally, the times of Salah are determined through observation. Thereafter, calculators may be designed to reproduce Salah times for the same area in which the observation took place.
The degree of depression of the sun (at dawn and dusk) and times of sunrise and sunset varies from region to region. Hence, it is incorrect to use observations and calculators from other regions when the two regions differ in this regard.
The EIC Salah Timetable is based on actual observations made in the UK. Further details on the topic and the criteria followed for the EIC Timetable can be found at http://moonsighting.com/prayer.html.