What is 'open pitlane'? How much as a helmet? I would not feel comfortable without insurance in case I destroy my 30k car. I took a look at some of the induction packages and they are like £300-500!
Open pit lane just means between 9:30am - 5pm you can go off and on track as you please. You could could do 500 laps or you could do 1, its up to you.
I just use my motorcycle helmet, you can get very good automotive helmets for reasonable money nowadays, not that you will need it.
I honestly wouldn't get hung up on insurance as the only time you would need it; is if you drive well beyond your limits and hit a barrier / armco (this rarely happens unless you drive like a nob). Nobody is going to crash into you unless you are reckless, in which case you will get black & white flagged and get a talking to and then get another chance.
If you are from Leicester then just do a evening at Donington, If you hit the barrier there then you have done well, unless lifting off through Crainer corner.