The major challenge here is whether they have properly balanced the weight of public interest.
They have used this generalised preference against disclosure:
There is a strong inherent interest for legal advice to be
exempt from disclosure as disclosure would prevent access to full and
frank legal advice.
FOI case law recognises a tendency to strong public interest in using the Section 42 exemption, but this seems to go further than the case law.
They have also stated:
The Case Evaluation document is a still current working
document, as some cases and the legal processes involved are ongoing.
But they don't discuss whether these cases could be redacted and the remaining information released.
I would ask for a review based on a reconsideration of the public interest test and consideration of whether a redacted document would meet the public interest test. I'd say your chances of getting any information are low but, depending on the quality of the review, the ICO may look at it (after a long delay).