Updated, December 2013
Short term insurance is now available for drivers as young as 17 but you need to have held your licence for at least 6 months. Tempcover Ltd is the insurer and has now changed its age restriction and is now providing coverage for under 21s.
Unfortunately, no insurance companies are willing to offer temporary insurance cover for 18 year olds.
Actually there's no insurer out there who will provide short term car insurance for young adults under 21 of age. If you go out with your bf a lot during the w/e, I suggest you get added to your bf insurance policy so that you are able to drive his car. Most of the times, when a female driver is added to an insurance policy, it has a positive effect by lowering the insurance quote, therefore your bf might benefit from a cheaper premium and you will enjoy the freedom of driving his car whenever you like, lawfully!
PS: You can get insurance for a minimum of one month though with eCar Pay As You Go Insurance as they insure people from the age of 17 and over for this type of insurance scheme.