I guess i'm lucky with my parents because I moved out to live with my boyfriend at 17. I have been living with him now for 6 months, and things are great! I'm a student, and I have a part-time job as a waitress doing a few hours a week, so that is enough to help with the food bills etc. I sympathise with you entirely...I stayed more and more time round my boyfriend's until I thought it would be more practical to move in!
The only advice I can really give you is to get your parents to trust and like your boyfriend. That's exactly what I had to do. I invited him over for dinner, got him talking to my parents, got him to invite my family over to his, got him to cook meals for them always help out, etc and luckily they love him, and really trust him, and know he'll ALWAYS take care of me. It took a lot of time, and believe me, it was a big step my mum letting me move out! My parents also really trust me too, and know that i'll always work hard, and nothing will affect that (which they knew anyway, given things that have happened to me in the past).
I think my parents realized that I was happier when I stayed over there, and in actual fact, my work improved, so I did well at my ASs!! It was also more practical for me staying over there, as it saved me carting my bags over there at weekends, once a week, and back again! So you have to also show your parents how responsible you are, and how happy you are with him, and how you really want to be together.
It also depends on how "sturdy" your relationship is I think. I was with my boyfriend a year before I moved out of home, and in that time, we had never had any serious arguments at all. We both felt it was definitely the right thing to do. You have to show that to your parents. Moving in is a big step in a relationship and you have to be sure its the right thing to do, and you have to show that to your parents. You have to show your parents how serious you are about your boyfriend, and how you will behave responsibly etc (which you seem to be doing already, paying for bills etc, so that shouldn't be a problem!!)
I hope this has helped you, as this is what i've learnt from my situation! But basically, show your parents how you'll behvae responsibly, and get them to know your boyfriend--and like him. If they are happy with your boyfriend, and know you're happy with him, then it shoudn't be too much of a problem (especially as you're 18, and your friends go off to uni at that age as you say!)
It isn't like your moving out to go to uni-you're moving out to live with someone else and that is different. It is natural for parents to be protective of their children under these circumstances, so even though they may not say "yes" at first--give it time.
I hope this as helped xx