i beleive refusal to provide a sample means you can be arrested and have a blood test at station. if you havn't had a drink then i cant see the problem in providing - you have nothing to hide and the police are only doing their job
The Drink Drive limit is hard to exactly work out, it depends on how big you are, how much you ate etc. Drinking x ammount can put some people over the limit whereas others would still be under it