You can only be cautioned if you agree to accept the caution, which effectively involves an admission of guilt. It's therefore difficult to see how you could succeed in any attempt to get a caution revoked.
Although a caution will be recorded by the Criminal Records Bureau, it doesn't create or form part of a criminal record. If a question on an application form asks 'Have you got a criminal record?', you can truthfully answer 'No'.
Because a caution isn't a conviction and doesn't create a criminal record, there is no provision, in law, for it to become spent. However, it's likely that you could lawfully answer 'No' to a question such as 'Have you ever been cautioned for a criminal offence?' once 5 years have elapsed, since this is the 'rehabilitation period' which applies to most non-custodial sentences upon conviction.
There are two reasons why you shouldn't have any problems entering the USA under the Visa Waiver Program:
Firstly, the form which you complete on the plane only asks about 'convictions for crimes of moral turpitude'. You weren't convicted and the crime doesn't seem to fall into the US definition of 'moral turpitude'.
Secondly, even if you go by the rules posted on the US embassy website (which are stricter than the requirements of the form on the plane), you can still use the Visa Waiver Program. This is because those rules state that anyone who has ever been arrested or convicted of an offence must apply for a visa. You've neither been arrested nor convicted, so you can still comply with the rules.
Chris