We have a problem in the uk with the fajr prayer time.its calculated that prayer time starts at 1,08 for imsak and sunrise is at 4,50 but dawrn starts at 3,20 .and dawrn is the start of light 1,5 hour before sunrise ,is it ok to do imsak at dawn because at 1,08 its still dark .local mosque fair starts at 1,08 but the fair prayer dont start till 4 am .please can help i am right to do imsak at darw tine 1,5 hours before sunrise

Praise
be to Allah.

The time when one must refrain from things that break the
fast (imsaak) is the breaking of the true dawn, as Allah, may He be exalted,
says (interpretation of the meaning):


“and eat and
drink until the white thread (light) of dawn appears to you distinct from
the black thread (darkness of night), then complete your Sawm (fast) till
the nightfall”


[al-Baqarah 2:187]. 

Al-Bukhaari (617) narrated from Ibn ‘Umar (may Allah be
pleased with him) that the Messenger of Allah (blessings and peace of Allah
be upon him) said: “Bilaal gives the adhaan at night, so eat and drink until
Ibn Umm Maktoom gives the adhaan.” He was a blind man who did not give the
adhaan until it was said to him: Dawn has come, dawn has come. 

It says in Fataawa al-Lajnah ad-Daa’imah (vol. 1,
10/283): 

The basic principle with regard to the fasting person
beginning and breaking his fast is the words of Allah, may He be exalted
(interpretation of the meaning): “and
eat and drink until the white thread (light) of dawn appears to you distinct
from the black thread (darkness of night), then complete your Saum (fast)
till the nightfall” [al-Baqarah
2:187]. Eating and
drinking are permissible until dawn breaks, which is the white thread that
Allah has made the end of the time when eating and drinking are permissible.
When the second (i.e., true) dawn appears, eating, drinking and other things
that break the fast become prohibited, and whoever drinks whilst hearing the
adhaan of Fajr, if the adhaan comes after the second dawn, must make up that
day’s fast, but if it comes before dawn, then he does not have to make it
up. End quote. 

Based on that, if a person is in a land where there is night
and day, then he must refrain from things that break the fast from the
breaking of dawn until sunset, regardless of whether the night is long or
short, and regardless of whether the time between dawn and sunrise is long
or short. For more information, please see the answers to questions no.
106527 and 2196. 

Secondly: 

There is nothing wrong with delaying Fajr prayer until
fifteen minutes before sunrise, because the time for Fajr prayer lasts from
the breaking of dawn until the sun rises, because the Prophet (blessings and
peace of Allah be upon him) said: “The time for Fajr is from dawn, so long
as the sun has not yet risen.” Narrated by Muslim (612). 

And Allah knows best.