Thanks for the answers.  Just to clarify:

Yes. The transport would have to wait until the next UK non-combat phase to unload the AA Gun, assuming it survives a German attack.

  In that scenario, the transport would be able to do the non-combat unload in the same turn as the conduct of combat?

Hi, just got the game and found the rulebook to be quite confusing so I began poking around various fansites and found the LHTR 2.0 which I found much less confusing.  However there were still a couple of this I wasn't 100% sure on.

1. Can you unload an anti-aircraft gun on amphibious assualt provided that it was loaded in the non combat move of a previous turn?
Assuming you can't and it has to unload next turn during non combat move provided the amphibious assault wa succesful.

2. On p 21 it states

Transports can move to friendly coastal territories and load or offload cargo, unless they loaded,
moved or offloaded during the combat move or combat phase.

Therefore can transports unload/load after combat but have not moved during combat phase (LHTR only?) e.g. after the scenario above, britain waits its next turn so it may unload its AA gun.
a. Germany attacks the transport, it misses and decides to submerge

b. Britain elects to stay in zone with subs and fights them on their turn winning the battle, may they now unload or load units to/from friendly territory in non combat move and thus be able to unload that AA gun.

Cheers.