How do I get all the information about my investigation from the Police after a NFA?

Although GDPR (General Data Protection Regulations) does not fully apply to police investigations you can make a Subject Access Request to the Police force that was investigating the accusation. Each force has its own web page where you will find the details to make this request. However, expect any response you do receive to be heavily redacted. The redactions will mainly be to remove details about other people.