I am not aware of any masjid in the UK that follows 15 degrees They either follow 18 degrees or the research of a small group ( 3-4 ) hizbul ulama in which their times fluctuates from lower than 12 degrees to over 18 degrees Let's leave out the argument of either 18 or 15 degrees and purely concentrate on the mushahada of hizbul ulama research, then we will understand that it is not permissible to follow their research Then the question would be what do the people of UK do ? The answer was given to the ulama of UK by mufti mahmood ul hassan gangohi and that was to use the 18 degrees method