“… when assessing competing public interests under FOIA the correct approach is to identify the actual harm or prejudice that the proposed disclosure would (or would be likely to or may) cause and the actual benefits its disclosure would (or would be likely to or may) confer or promote. This … requires an appropriately detailed identification of, proof, explanation and examination of both (a) the harm or prejudice, and (b) benefits that the proposed disclosure of the relevant material in respect of which the exemption is claimed would (or would be likely to or may) cause or promote.”