At a basic level all you need is your road car and a helmet (available from ash on here for about £40) - these don't usually need to be fia approved on trackdays so a motorbike one will do. Open face are also usually allowed (in closed-roof cars), but no dodgy plastic BMX ones from when you were 5
Car should be in good condition - I don't know of anyone who's been asked to show an MOT, and some cars on trackdays don't have them anyway, but it should be well-maintained and safe. It's your life at risk, as well as others, if it breaks
Brakes will take a pounding - be prepared to shell out for new pads afterwards if you're hard on the brakes in normal road-driving (i went through a set of pads at croft, but possibly because they were new, not bedded in properly and i worked them too hard - in my nova they seem to last forever (unless ash is driving ) - track-driving puts more stress on every part of the car, so while there may not be any failures on the day you should be aware that the wear rate of things such as suspension compenents will be sped up.
Tyres - can use your normal road tyres, tho cornering forces will scrub off the edges. Some people use ebay to buy second-hand rims with worn out tyres to use, a trackday will allow slicks so tyres below the mot depth line will be fine, but you should check they're not dangerously low - if the tyre bursts at 110mph, it's going to get messy!
Insurance - not compulsory - only 4 or 5 people had it at croft. From CCI / Egger Lawson - for a £7k value car = £70 premium and £700 excess on any claim. That covers your car only, so it's not like your usual road insurance - even if the accident isn't your fault, if your car is damaged but you have no cover then it's no money for you. In reverse, your insurance does not cover the 3 porsches and a ropey old rallye you wiped out swerving to let that nice pete-rallye by. Also does not cover mechanical failure. I'm sure Kev can provide Chris Yemm's phone number......
It really is great fun, you'll never get the chance to drive you car like this on the roads.
If i've forgotten anything (like cost of replacement mower blades) then feel free to add on.......
________________________________________"Talking about badgers is life. Anything before or after is just waiting" Steve McBadger
Transversa aurigatio transcurro arvae.