If your going to do proper racing though you'll need to have a proper helmet with the right safety specs.
Ok, but a motorbike helmet will be fine for trackdays yea?
Hmm I can see a car one taking more. Realistically a bike one is more for maybe one impact, then scraping on the group. In a car crash your head has much more to hit, especially in a roll.
Snell helmets have nomex lining for when your burning in the car and they are also tested to with stand hitting the roll bar alot.
There is a mimimum size for the eye hole in a motorcycle helmet but snell helmets are allowed smaller.
If your going to do any racing you really need a helmet thats Snell SA2005 Spec. The MSA has now outlawed Blue Sticker BS helmets so watch out for those on ebay, im having to get a new one for 2010 because of this.
Someone mentioned SA2000, that will only be legal for a minimum of 1 to 2 years as SA2010 is coming into play end of next season.
Check out V2Sport.com and Hedtec.co.uk. Affordable SA2005 helmets which will do you for motorsport and trackdays.
Bare in mind also, that at present yes Motorbike helmets are allowed on trackdays, but there is talk of bringing them into line with the MSA and having ot have proper standards.
My advice is get a decent one now which will last you.
No Worries. Those V2 Sport and Hedtec ones are worth considering. Im going to have a nosey at the autosport show see how good the quality is.