The rules in the UK have changed somewhat over the years; it used to be that flashing lights were only permitted in addition to a constant lamp.
Now its fine to only use flashing lights - Rule 60 of the highway code:
"At night your cycle MUST have white front and red rear lights lit. It MUST also be fitted with a red rear reflector (and amber pedal reflectors, if manufactured after 1/10/85). White front reflectors and spoke reflectors will also help you to be seen. Flashing lights are permitted but it is recommended that cyclists who are riding in areas without street lighting use a steady front lamp."
With the insane brightness now possible I wish they had to meet the same light pattern rules car headlights do. I think that would help the flashing being so distracting.
A very bright bike light makes it much more comfortable to ride a bike at night, you need to compete with car headlights so that you can see the surface of the road in front of you. Cars don't need to worry about the road conditions as much as cyclists since if they hit a pothole or a stick, it's no big deal.
I'm hoping cars (and bikes) evolve over the next few years into less blinding headlights. The infatuation we have with projectors makes even halogen lights blinding. I see some newer Acuras coming with multiple LED projectors on each side lately, which ought to help (so long as overall output is constant, and each projector is dimmer)
Now its fine to only use flashing lights - Rule 60 of the highway code:
"At night your cycle MUST have white front and red rear lights lit. It MUST also be fitted with a red rear reflector (and amber pedal reflectors, if manufactured after 1/10/85). White front reflectors and spoke reflectors will also help you to be seen. Flashing lights are permitted but it is recommended that cyclists who are riding in areas without street lighting use a steady front lamp."