This answer to this question states that the onus is on the defendent to prove they were not going to drive while they were unfit. Leaving aside the question about the flipped burden of proof, does this leave somebody open to the possibility of prosecution for having a few drinks at a place where they intend to spend the night, bedding down while over the limit then, for some reason, coming to the attention of the law and being found to be over the limit?
For example, Mr and Mrs BWFC park their van for the night in a place where they are fully permitted to park it and proceed to have a few local ales. They go to bed and in the night somebody drives into their parked van. The police attend and get out the breathalyser and ask everybody to give a sample. They decide that because Mr and Mrs BWFC were in the van at the time, they have to provide a sample. Mr BWFC's sample is over the legal limit and the wheels of justice start to grind.
Under section 4, subsection 3 of the Road Traffic Act 1988 as cited in the linked answer, it seems to me that an offence has been committed but the likelihood of charges would be low. The circumstances may be a little contrived in this case but they are not so unrealistic that the situation would never come up.