Hey jammiedodger. Good luck for moving out-I left home at 17 and love living independently! It's a lot to consider though and you have to be very strict with yourself (I am a complete homebody so I don't go out much anyway!)
We (me and my partner) live in a 2-bed terrace and pay quite a low rent as our house belongs to a housing association. It's worth getting your name on association lists if you can, as the rents are usually very minimal-ours is �63/week.
If you are a student you are exempt from council tax, but you have to let them know, and provide proof of your registration at college/uni. If only one of you is a student, the other will only pay 75% council tax.
So as well as rent and CT, we pay water rates (�13/month), electric (�20/month), gas (�30/month-less in Summer), fixed line phone (�10/month), Broadband (�14/month), TV licence (�14/month), mobile phone (�30/month). We spend around �200/month on food, but we don't go out a lot so that saves money.
Also consider that there will be a lot of compromising to do if you're moving in with your b/f: moving out is a whole different experience when you find you're still hving to consider someone else!
Hope that helps and lots of luck for your adventure!
Lisa x