Yes, the planes from Southern Italy can scramble to that seazone. as by the rules
"They can also be scrambled to resist amphibious
assaults from adjacent sea zones, whether or not the
territory being assaulted is the territory containing the
air base. They may defend against the enemy ships
conducting the amphibious assault even if friendly ships
are not present. "
The British land units may not be unloaded before the seazone has been cleared of the enemy (scrambling) units.
As friendly powers never fight together when attacking, only the British units participate in the sea battle. Scrambling is a defensive move, so refer to the scrambling planes’ defense values and abilities when resolving combat.
The American units only watch the scenery, as friendly powers never attack together. As Britain in your scenario is the attacker, the British transports may retreat, if possible.