For a Road Traffic offence, you would be tested at the police station, after arrest, on a more sophisticated breath machine. Your lower of two readings is then taken and a prosecution will follow if your lowest reading is over the threshold. You will be in court in a matter of days if you provide a sample over the limit. A minimum of a 12 month ban can be expected.
For pilots, etc., my understanding is that a blood sample must be used due to the lower threshold for prosecution. The time taken for analysis is the reason for the delay in processing charges, hence a bail pending the results. However, a failure on the station breath test would provide a good indication of the likely outcome of the blood test......
The right to bail is presumed, reasons need to be found not to grant it.