Actually you only receive a caution if you admit to the offence, and the police do not feel it is necessarily in the public interest to refer the matter for prosecution - using the cps. The caution is recorded against your name for 2 years (in most cases). If you reoffend within that time, it can be brought up again in court.
You can only be charged if the cps think there is a "better than reasonable prospect of success" of your being found guilty.
Remember, for any criminal offence to have ocurred, it is necessary to prove three things: (a) That you did it (b) That it was an intentional act and (c) That you knew what you were doing was illegal. The latter is called Mens Rea, meaning "bad intention". Without being able to prove it, you have committed no crime.
For example, if you were drunk and disorderly then you could defend your conduct by saying that your drinks were "spiked". In that way you did not intend to become drunk, and it is for the police to prove otherwise.