RoSPA recommends that cyclists wear a cycle helmet that meets a recognised
safety standard. Cycle helmets, when correctly worn, are effective in
reducing the risk of receiving major head or brain injuries in an accident.
They do not guarantee protection, nor prevent accidents from happening in
the first place, but wearing a cycle helmet is a simple, low cost and
effective way that individual cyclists can protect themselves.

A cycle helmet cushions the head in a fall, providing a last line of defence
between your head and the ground. It reduces the force of an impact before
it reaches your head and brain. The hard outer shell spreads the force of a
blow over a wider area than the initial impact site.

Choosing a cycle helmet:

Try the helmet on before buying it
Make sure you like the type and style
Check it has a CE mark and meets at least one recognised Standard: BS
EN1078:1997 (European Standard) Snell B.95 (American Standard)
Make sure it fits comfortably and securely
Check that the straps are easy to do up and adjust
Ensure it stays in place on the head when the straps are fastened
Make sure it does not obstruct vision
Ensure it does not cover the ears
Check that it is well ventilated
Ensure it comes with clear advice for the user.

Remember:

Helmets do not prevent accidents. An accident can still be very serious,
even when wearing a helmet. So be just as careful, look around for traffic,
dress brightly and follow the rules of the road.

Source: http://www.rospa.com/faqs/detail.aspx?faq=364