You may need to do this through hostile witnesses - or in circumstances which could work against you. Getting information from the police is particularly difficult . Never ask them for documentation or anything about productions, it not only alerts them to what you are up to - and then some officer may somehow "lose" the very evidence you need. If you really have to do this, go through the solicitor.
If there is a specific piece of information you need, you will readily see how the case you are working on is a good reason for asking for that information. However, if you say that to whoever you want the information form you will reveal what you are doing. That's dangerous for you.
Think laterally -
Most pieces of information can be found through some other means of approach. First, define precisely exactly what the information is that you need verified. Next, write down as many reasons why other people might want that piece of information. Imagine you are an entirely different kind of person - why would you need this information? Then work up a "back-story" using one of these reasons and go for it.
For example, you want to check on the CCTV owned by the local council. You ask them if you can see it because you have been told that your missing dog was seen in that area at that time. They are more likely to let you see it than if you tell them you are working to get a convict free.