Buy a cheap one, like a 661 Strike from CRC at £40. Measure your head, order and then change if it’s not right. If you’re just starting to give DH riding a go, then it’s not worth spending more on a super expensive helmet. It’s questionable whether the more expensive helmets offer any greater degree of protection anyway, perhaps the ones designtated for MX (i.e. motorised use) do, but you’re not going to be going fast enough to need that extra protection.
Most of the premium comes from use of materials and design, e.g. a carbon Troy Lee is £300 because a)it’s Carbon and b)it’s a Troy Lee. In reality you’re head’s no safer because it cost 7 times more than the 661 cited above.
You should also get some body armour, again, nothing too expensive. This will not stop you from breaking bones but it will save your skin from being ripped to pieces when you crash (and you will crash!). A jacket with spine guard, knee/shin pads, goggles – these are essential; the last thing you want is crap in your eye – and probably dual ply tyres. Maybe get the body armour from eBay? Best not to get a helmet from there though.
Let us know if you need any other advice on getting started and above all have fun.