[Grumpy]

I will second Sports Bike Shop. Not always the cheapest, but the staff are knowledgeable and friendly. Good choice of makes backed with stock levels to match, plus as mentioned, a user friendly returns policy.
Lastly, they sell genuine products, not cloned look alike from China.
Be wary of ultra cheap helmets with known brand labels. They may look the same, but are not made to the same std. I think it was MCN ( not sure on the publication) who earlier this year purchased a cheap branded helmet off the net, then took the helmet to the manufacturer. They tested the helmet, it failed the euro tests by miles.