First of all they can prosecute in terms of S4 of The Road Traffic Act 1988.They then have to lead evidence that the person is impaired and that the manner of driving is defective.
The second way is by charging them with S5A of the Road Traffic Act 1988.The offence is having a certain level of a particular prescribed drug in your system. This can apply both to illegal and prescription drugs, for example if you are found to have cocaine, heroin, cannabis, ecstasy or various other illegal drugs then you will have a case to answer in the court. As a result of various prescription medicines that are routinely prescribed there are defences in relation to prescription drugs.
When passing the legislation the Government listed approximately 50 drugs that were illegal and if you have these at a certain level in your blood stream you are guilty of an offence. For example Cocaine which breaks down very quickly in the blood stream leaves a by-product in the blood which is also prescribed by the S5A of the Road Traffic Act 1988.
If you have provided a blood sample to the Police you should be offered a duplicate blood sample. You should get that analysed as soon as possible. Remember to keep it in the fridge as the blood sample will degrade if it is not kept chilled.