At the time the Act became law the CAA issued guidance that suggested that all breathalysers issued to UK police had been tested and shown to be capable of measuring the lower level with sufficient accuracy for preliminary tests. The same guidance also said that the station evidence would normally be blood or urine although the Act does allow for breath. That may have changed since 2003 when the guidance was issued.
There are two different offences in the Act, being unfit for duty and exceeding the prescribed limit. The first is quite difficult to prove so the second is the one usually used. It is quite possible to be over the prescribed limit and committing an offence but be neither noticeably drunk or unfit for duty.
The offence is committing an Aviation Function or an activity that is ancillary to an aviation function whilst over the prescribed limit. What is an ancillary function is not totally clear but the guidance from the CAA was that it would include pre-flight briefing and post flight filing of reports in connection with the flight.