srw
It's a bit more complicated than that...
You're looking in the wrong place.This thread was on my mind when I was in All Terrain Cycles in Saltaire idly browsing my lunchtime away. I looked at helmets for the first and a last time in my life, the first one I picked up was £129.99, the second a Giro Aeon (or summat) about £180. Wowee! I had no idea, I though they were about £20 tops.
£10 if you've got a small head - http://www.halfords.com/cycling/bike-helmets/adult-bike-helmets/trax-furnace-bike-helmet-54-58cm
Or £10 half price - http://www.halfords.com/cycling/bike-helmets/adult-bike-helmets/bell-avanti-bike-helmet-54-61cm
Both meeting EN1078. If I remember correctly, the last one I bought cost £40 from an LBS.