One, the Italians would have to control Gibraltar at the beginning of their turn. I also think the UK could possibly roadblock the Italians in SZ 104, though theoretically German aircraft could take care of a lone destroyer. If Italy comes out I feel like they are kind of taking their eye off the ball in Africa, which as the UK I would welcome. It is only worth it to me if it helps guarantee a successful Sea-Lion.
Using your strategy, I am actually considering next game turn 1 buy for the UK: 1 carrier, 1 destroyer, and an infantry. (It kind of depends on what Germany buys)
This will give London the following units to defend for possible turn 2 Sea Lion: 6 infantry, 1 tank, 1 tac bomber (french) plus possibly 1 infantry and 1 tank from Quebec.
To assault London turn 2 Germany will have to go through the following: 2 carriers, 6 fighters, 1 destroyer, 1 cruiser (possibly a second depending on battle in SZ 91). On top of that, depending on the dice roll, I oftentimes have a battleship or cruiser or destroyer that survives his assault in the Atlantic.
This strategy depends largely on the following:
1. What Germany builds turn 1
2. Where Germany positions their aircraft at the end of turn 1
3. How much of my fleet, if any, survives the German turn 1 assault