This is a cracking deal - a Giro Havoc helmet for £20 from Halfords. I've just got back from picking mine up and it's amazing quality. Normally for a £20 bike helmet you get an unknown brand's basic model. Here you can get a top model from a top make.
It's still £49.99 in store, so make sure you reserve it online and you'll pay £20 in store. On site, it just says that they have the small model available, but when I went in, I asked if I could have a medium and that wasn't a problem at all.