First, an explanation of the offences:
There are two separate offences involving vehicles and excess alcohol. One is driving whilst over the prescribed limit (which carries a mandatory minimum of twelve months disqualification). The other is being “in charge” of a vehicle whilst over the prescribed limit (this carries no mandatory disqualification but a minimum of ten penalty points on conviction). Both these offences involve a roadside breath test (which is just a screening test to establish if a more accurate test is justified) and a test back at the nick which is more accurate and which is used for evidential purposes.
There are also two other offences, namely of being “unfit to drive through drink (or drugs)”. These are similar (driving and “in charge”) and carry the same penalties. However, these are not evidenced by breath/blood/urine tests but by subjective assessment of the driver’s fitness to drive.
The drink-driving legislation (that is, the legislation describing the actual procedure) is complex and involved. The routine that has to be followed both at the roadside and at the nick is carefully prescribed and must be followed to the letter. Various requests for information must be made of the driver, various warnings must be given to him and the physical process of taking the test(s) is also rigid. It is easy for a competent solicitor or barrister to notice if any of the various steps have been omitted or carried out incorrectly. I haven’t seen a set for a while but the paperwork for processing a suspect runs to about 30 to 35 steps as far as I can recall. A deficiency or shortcoming in any one of these can jeopardise a prosecution.
Of course we don’t know what happened here. When the prosecution “offers no evidence” in a trial it does not mean they have no evidence (if they had none the matter would not have made it to court in the first place). If is the formal way of discontinuing proceedings. They may do so, for example, because they have laid alternative charges and, having decided on which one they were going to pursue, they “offer no evidence” on the other. They may also do so when they realise that there is a deficiency in their case and that there is no prospect of a conviction. The deficiency may have not become apparent until the court hearing. I suspect that is what may have happened here.
“On the other hand why would you appeal if you know you are guilty.”
By “appeal” I presume you mean plead not guilty. Many defendants who have committed offences plead not guilty for a variety of reasons. The burden is on the prosecution to prove their guilt. The defendant may believe that there is insufficient evidence to convict them, or that witnesses with vital evidence may not turn up to give it. They may also have been advised that there is a shortcoming in the procedures adopted to gather evidence so they plead not guilty and let the court decide on the matter.