UK legal system. Can you contact the police for information about a crime and legal sentencing or is it protected information?

Anyone can attend a court hearing - there are public galleries.
Once someone has been convicted in a court of law, that information is available to the public. You can usually find details in the local newspapers.
The police won't tell you - that's not their job.

However, if you have information that someone has committed a crime, or lied to the police, and it's real, genuine evidence, rather than just your opinion or guesswork, you can write to the police in the are which would have dealt with it and tell them what you know.

The police do not sentence people the courts do and no one is going to discuss with you about a named person(s) in any case. There is a Government website where you can read about the law https://www.gov.uk/government/topics/law-and-the-justice-system and law libraries where you can read case studies

No, you can't. The only people who can are trust officials such as the High courts, a government spy agancy or an attorney/lawyer/detective with a high reputation.

Only some information will be released to the public via media but everything else is private.

The police won't share that information with you - not that the police have anything to do with sentencing in the first place.

Details of completed court cases are published in local newspapers. If you have a legitimate reason to know you can also apply directly to the court for the information.