What is the correct time to follow for Fajr and Asr prayers in Glasgow, especially with the differing times between mosques, given that Imsak begins at 2:48 AM in some mosques and 3:30 AM in others, knowing that sunrise is at 4:37 AM?

If it is difficult to ascertain the prayer time through the Shariah-prescribed signs, then do not pray until you are certain that the time has begun or have a strong presumption [that it has]. The same applies to abstaining [from food and drink] and breaking the fast; do not abstain or break your fast until you are certain or have a strong presumption.