h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This ...
http://www.opsi.gov.uk/si/si2005/20052559.htm
... is an amendment to this ...
http://www.opsi.gov.uk/SI/si1989/Uksi_19891796_en_1.htm

As I understand it, flashing front and rear lights are now legal as long as they "produce a light with an intensity of not less than 4 candelas."

(And furthermore, as long as they are only capable of flashing, they don't need "an approval mark or a British Standard mark". Perhaps this is because of the 4 candelas minimum, which applies to them instead.)

For a clear and concise explanation of the current law which is confusing see the Cyclists' Touring Club page on the subject:

http://www.ctc.org.uk/DesktopDefault.aspx?TabID=4071

The more one looks at this subject, the more confused one becomes!

Yes it is important. If you are involved in an accident, the lights on your bike will be examined.

Many MPs are lawyers. A cynic might conclude that the confusing state of UK legislation is no accident, since the primary purpose of parliamentary legislation seems to be providing employment for lawyers.