I am a late sleeper and more often than not I have insomnia and I can tell you that at the end of June during the longest days of the year, the first light can be seen at around 4:30 AM with full day light at 5 AM so like I said if we move to GMT on top of using winter time all year round, the first light will be seen at around 2:30 AM with a full day light at 3 AM.
So If they moved to an all year round winter time I would be ok with it but GMT? I don't think it would be a good idea, in my opinion even if we are naturally closer to it than to CET.
I know that there's already light before 5 AM, but the sun rises at 5:30. It's also still light at 11 PM, long after the sun has set.