Legal jargon

[Nanny67]

If a letter from the police says "there is sufficient evidence for a successful prosecution but it's not in the publics best interests" what does that mean? Does that mean the person is guilty?

no. It means what it says, it's up to the court to decide if someone is guilty, but in this instance the police are not going to pursue it.

It means it’s likely they could be found guilty (but not definitely) but that it wouldn’t benefit the public to prosecute them. You can’t say if someone is guilty or not with having a full trial.
If you wanted a prosecution you could always pursue it privately. If you didn’t - then be relieved!

Agree with previous answers. The police believe they have enough evidence to get a guilty verdict, although they can't be sure without taking it to court, but they don't think it is in the public interest to pursue it. If you want to know more about the public interest test for prosecutions, see pages 7-10 of this document.

This sometimes happens when the police have enough evidence to prosecute, but the cost of bringing that prosecution to court (If there is a likely fine) will outweigh the fine itself, or if the person is unlikely to pay (in my experience).

Transport workers like me have occasional contact with the British Transport Police. The courts and police are both very cash-strapped now. My neighbourhood policing team who looks after my group of stations tell me that something like fare evasion by a drug user may be dropped "in the public interest" because the likely cost and fines outweigh each other, and the plaintiff is unlikely to pay (hence why fare evasion is high on my workplace, the Tube, because it doesn't make financial sense to pursue).

If there is anything more serious, like an assault, there may be a public interest to pursue the matter, but I have seen cases in the past of even assaults being dropped.

Thank you. It's regarding an arrest for indecent images.

There are any number of reasons why the public interest (second part of the two-part ‘full code test’) can be found against by police, CPS, HSE, etc. VanGoghsLeftEar (excellent name BTW) has given good examples.

Not every crime should be prosecuted.