Read Henley on original case documents.
I think there were unseen photographs as well.
Why spend all that money when the documents and images could have been released?
Who took the unseen photographs?
Why wasn't Bamber's lawyer, Joe Stone QC, challenging refusal of access to these photos at the Judicial Review?
To answer your question "Why spend all that money when the documents and images could have been released?" - a public body should follow their own legal advice, rather than capitulating to a request to save money on defending a Judicial Review.
If they capitulate to Bamber's lawyers, they would have to do the same with everyone, everytime someone writes in to request something.