It is the going rate for someone of you age. It will depend on a lot of other factors as well. If you do the Pass Plus you will probably get a good reduction.
In view of the fact that a lot of people of you age are involved in accidents, Insurance Companies charge a higher premium.
I paid �124 a year for insurance when I was 18 - but that was in 1980 (in my mk2 escort). Blame your mates - the high accident/claim rate for 18 year olds justifys the cost.