This is my first deal, and a bit of a re-post, but hopefully someone will find it useful...
Halfords have a range of Giro helmets at insane prices. The one in the link is a Giro Stylus, which RRPs at £50 and the best price elsewhere is around £45. But if you reserve online at Halfords it's just £20.
This is an amazing helmet - it looks great, there's loads of ventilation and it's got great reviews for safety. I used to hate wearing my old helmet and would often go without - now I wear this all the time (well, only when I'm riding!).
Please note: If you go into the shop it will be on the shelves at £50, so you have to reserve it online to get this price.