Dazzling bicycle lights

[Grahamm]

you must have a fixed light (constant) by law on a bike

Nope, the flashing ones have been legal on bicycles since (I think) 2004.

Highway Code Paragraph 60

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.
Law RVLR regs 13, 18 & 24

the flashing ones do catch the eye

True, but I do recommend the ones that flash very rapidly rather than the ones with a distinct On-Off-On-Off or, even worse, Flash-Flash-Flash-Off pattern because during the "off" phase it's very easy to lose track of the bicycle since your eye is looking for the flashing light, rather than the cycle it's attached to.