If you refuse or do not provide a specimen of breath then you will be prosecuted regardless of whether you drove or not. All the police would need to show is that they had suspicions that you had or were going to drive.
The defences are very limited and only if you were physically incapable of providing a specimen due to a medical reason would you have a potential defence. If this is the case then it is imperative that you mention this during the interview.
The starting sentence is an 18 month disqualification and this can also attract a community order or even in some circumstances a custodial sentence.