There is probably something I am fundamentally misunderstanding here. I live in the UK. In winter I am familiar with the timezone being called GMT. 6:00am is definitely in the morning (and I am usually asleep).

I'm not one hundred percent sure that could be right... If you see a flash it means an action potential in a rod of cone fired. It's very hard to believe that a cosmic ray is going to, say, cause a potential difference sufficent to fire an action potential: you'd need some kind of interaction with the cells themselves opening a sodium channel or something. I guess when cells die they tend to fire off a bit, but to actually SEE a flash you'd need to have many of these things going on simultaneously. In general, ionizing radiation is bad. But just how much more damaging are cosmic rays?