I can’t imagine why some details would still be covered under the Official Secrets Act.

The war ended 66 years ago, the Allies won against a regime that no longer exists. I’d wager that all the planners are dead so they don’t need protecting.
No secret weapons were used, so it’s probably not anything technical.

Perhaps something to do with French internal matters?
Or perhaps the raid was a dress rehearsal for something similar or a diversion.