U-Boat in SZ21. UK, on its turn, combat moves Lt CV with fighter and a light cruiser to SZ21. UK can attack the U-Boat immediately as CV with a plane always has MAT. The light cruiser can also attack, but the U-Boat gets 1st strike because the UK does not have a Destroyer.
I believe this is also incorrect. Carriers cannot move before your plane goes on MAP. The reason is this movement by planes is a form of combat movement. Your carrier is not moving into combat movement. When you refer to the plane on the carrier as always having MAP, that is correct, if a sub tries to go through the SZ.
The light cruiser cannot attack if the plane is on MAP and the sub is in that SZ. Only a destroyer paired with the plane can attack.