From here http://www.ctc.org.uk/desktopdefault.aspx?tabid=4071
Flashers
Thanks to the enactment of Statutory Instrument 2005 No. 2559: on 23rd October 2005, it finally became legal to have a flashing light on a pedal cycle. Even better: it became possible for a flashing light to be approved, meaning no other light would be needed in that position. And since BS6102/3 does not yet cater for flashing (but is likely to be amended to do so quite soon), approval is for the time being, granted simply on the basis of brightness.
Because DfT very much prefer anything that possibly can be evaluated against a proper technical standard, so to be evaluated: any flashing lamp that is also capable of emitting a steady light is approved only if it conforms with BS6102/3 when switched to steady mode. Since most (probably all) flashing lights do also have a steady mode, and since none of their manufacturers can be bothered to test and mark them to the pernickety standards of one small country on the fringes of Europe, it’s unlikely that any flashing light actually qualifies for approval. But since it became theoretically legal to ride a bike with only flashing lights on it, the Police are nowadays no more likely to quibble its legal status than one equipped with steady lights – unless they’re rather dim or involved in an accident of course.