Be wary of warnings!
Theres so much boll*cks talked about helmets its untrue. You dont need a 'car' helmet and a cheap Chinese Snell approved helmet will be massively inferior to a good Arai or Shoei,
You will also probably find that either an Shoei or Arai will fit you better than anything else. Unfortunately the ECE/EU testing has put paid to BS6658-A testing now, but any Arai/Shoei with an ACU gold sticker will be to the same standard (and itll far exceed ECE requirements).
A reasonable and cheap ECE helmet will provide all of the cover that youre going to need on a trackday. Id treat '£30 helmet for a £30 head' remarks with contempt, although theyre often repeated.
Id also advise against being swayed by cheaper, but very expensive for what they are, lids from name who you know, such as OMP and Sparco. If theyre only ECE approved then youre paying £100+ for a £30 helmet.
No trackdays require a fireproof lined helmet and unless youre wearing matching socks, underwear and a min double layer Nomex suit with balaclava, gloves and boots, then there is absolutely no benefit to you (and even if you are theres no benefit as youre wearing a Nomex balaclava already).
Finally, if anyone tells you that 'car helmets' are designed to withstand impacts but 'bike' helmets are only designed to resist sliding down a road, then politely excuse yourself, realise that theyre a fool and carry on as though they hadnt spoken! [

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