Well, if you want something light, then I would go for a Masuri Ti. Not sure how well ventilated these are though... I know an Ayrtek Premier with a Ti Grill is a decent weight and has good ventilation for that price though! Offers more protection too, but not quite as light as the Masuri Ti.
I think I saw some Slazenger Ti Helmets on the Ebay Bargains thread going for around less than £50, which are very similar to Masuri helmets, so maybe that would be a good place to look, as you would be saving £50 for other stuff!
I would probably go for the Ayrtek taking all these things into account, as I want the best protection I can get!
Hope this helps!