Bez wrote: ... Fair enough (I was thinking of S5 of course), but I suspect (happy to be proven wrong) that S4 prosecutions are rare as hen's teeth given the existence of S5, Plus of course other offences like S3A refer to the numerical limit.
Point still stands, though: from a prosecution point of view, relying on a measurement a) is not inherently better, b) can often be worse, and c) given a starting point of 100% cannot possibly be better in this case.
Numbers cut both ways, of course. Taking your breathalyser example, that's what I meant when I wrote "They can then be offered breath tests at a police station which can be used in evidence either way." Blow positive on a police station machine and that's that. Blow negative and even if you are falling about drunk, it's going to take a lot to get the case to court. A clue here is that they have now and at long last introduced tests for the people affected by other substances: the people in my day who were falling about but blew negative.