I'm not a lawyer, etc. In many jurisdictions you have a legal obligation to provide a sample for testing of intoxication when driving. (You consent to this by agreeing to drive.) In those jurisdictions, refusal to provide a sample is the same as intoxicated driving. I believe the UK is such a jurisdiction. That is to say, the offence is worded in a way where refusing to provide a sample has the same punishment as driving while intoxicated. So you may end up convicted of the drunk driving offence anyway. But you will also appear remorseless, like you were trying to game the system to get away with drunk driving. Which will likely result in a harsher sentence than if you showed contrition.