england-and-wales
The CPS decide who to prosecute, not the police
In general, despite the mechanisms discussed below, it is not advisable to ever rely on such a promise from the police.
The police have no decision-making power and so cannot make a promise of that nature. Even if such a promise is made in good faith, it cannot be enforced.
However, if someone relies on a police promise and later gets charged, the court may deem this an abuse of process. In practice, this is rare and must show a significant and unfair reliance on the promise (see: R v Croydon Justices, Ex parte Dean [1993]). Even when this does happen, it does not necessarily mean that there will be no charges. Instead, this particular piece of evidence may simply be excluded from the proceedings under the Police and Criminal Evidence Act (PACE). There are two relevant grounds for exclusion under these circumstances: 1) unfairness, and 2) unreliability.
Beginning with unfairness, refer to s78(1) of PACE:
... the court may refuse to allow evidence on which the prosecution proposes to rely to be given if it appears to the court that ... the admission of the evidence would have such an adverse effect on the fairness of the proceedings that the court ought not to admit it
Similarly, regarding unreliability, this is considered in s76(2) of PACE:
If ... it is represented to the court that the confession was or may have been obtained—
... (b) in consequence of anything said or done which was likely, in the circumstances existing at the time, to render unreliable any confession which might be made by him in consequence thereof,
the court shall not allow the confession to be given in evidence ...
As a result, confessions that are obtained in circumstances that render them unreliable may not be used in evidence unless the prosecution can show that they are not unreliable beyond all reasonable doubt.
