Being 18 wont bring your quote down I'm afraid, and it wont go down until you probably reach around 21 (significantly anyway).
I would, from personal experience, strongly discourage you from becoming a Named Driver. You do not accrue any "No-Claims" bonus that way, and you will pay for that in the long run.
I strongly advise you to just fork out the cash for the year (maybe convince your parents to pay the annual charge and you just pay them back in monthly instalments?) and see how that goes. Once you've completed the year, go and shop around with the year's no claims you've got to hand, it'll go down slightly but not a lot.
One main thing, remember to always include modifications to the car if you have any. Many insurers will use this as an excuse (any excuse will do!) not to pay out in the event of a claim.
I live in Edinburgh, and we have a lot of Boy Racers around. Many a time (okay once) I have seen them crash their car, and begin ripping off their spoiler, exhaust, etc. because they've not declared them.
I know this doesn't solve your financial problems, but believe me, in the long run it will. I'm 20 now, and I'm still paying what I paid 2 years ago as a named driver, purely because I never got any no-claims to lower the cost!