@loki17 your example is somewhat improbable, as the UK is not at war with Italy in 1939 and can only declare war to a nation has declared war on another nation during the game (G.B. NRS v3.5).
However, in the event the Italy declared war before the Britsh turn, if the British attack the Italian fleet on 1939 turn one, this should be per naval combat rules (rule 9.6 and Table 9-4), but the submarines may decline combat unless you place aircraft on MAP (8.9).
In the box on page 37, it specifies that the attacking player will declare if he puts aircraft on MAP first and then the defending player will declare what he does with his subs.
However, if the Italians want to use their subs for defense, then I don’t see why you would need MAP, as they will take part in the naval combat. As @GeneralHandGrenade mentionned above “He has chosen top be in the fight and is open to being hit”.
However, rule 8.9 regarding submarines mentions:
“Note: This effectively means that to kill submarines you either have to wait for them to Attack, kill them during convoy-raiding or hunt them using your aircraft on Maritime Air Patrol (paired with destroyers).”
Considering the previous comments, I feel that this note is incomplete, as you can also kill sumarines if the defending player chooses to have them take part in naval combat and does not submerge them before they are destroyed.