If Italy had invaded early enough…say prior to 1941…I doubt that any effort by the UK could have retaken it.
They were simply unable to manage Axis air superiority to make a landing in strength in the central Mediterranean, especially after the fall of France.
The longer the Axis waited, however, the harder the invasion was going to be. The civilian populace would be more apt to resist, and there were more British military personnel being sent to the island. Italian deficiencies in naval armor, naval radar, and joint air/naval operations also became more apparent to the Royal Navy as the war progressed.
The inevitable German invasion of Russia also ensured that German support would be limited after mid 1941.
That is about the point in time that I would tilt the scale from the Axis to the Allies. There was no hope of the UK retaking Malta had it been lost prior to that point. Likewise, after that point, there was no chance the Axis could take the island without incurring enough casualties to make it a pyhrric victory.