The offence of drunk in charge of a motor vehicle is an offence which is very easy to commit but has a statutory defence which is very strong. If you are in or near to your vehicle whilst intoxicated you are drunk in charge of the vehicle and therefore could face a prosecution.
Factors such as presence of car keys and the engine running all have a bearing upon a case such as this, but the real key to defending a case is intention to drive. If you can show that despite being drunk in charge of the vehicle at no point were you intending to drive, then you can utilise this defence.
You will need to convince the court of your lack of intention and you will need to show when it was your intention to drive next. An expert will be required to carry out a calculation to show that at the time you were intending to drive next you would not be intoxicated and if this is proven to the satisfaction of the court a not guilty verdict should be returned.