Of course but that's not the issue at hand, the question is whether the AA check-in agent would see this as a trip to Italy (and thus require the negative test at LAX) or a trip to the UK (since the connecting flight LHR-MXP doesn't depart till +2 days later).

After Oct. 4 no negative test is required to enter/transit the UK.

Assuming the OP makes it to the UK, I'm guessing they're going to hit bad traffic and miss their LHR-MXP flight, thus making any Italian entry requirements a moot point.