I'm on an Avios booking next August from MIL to LAX. The LHR-LAX leaves 9:55am. System would not let me book the 7:25am LIN-LHR leg because it's scheduled to arrive at 8:30, I guess 5 minutes shy of MCT, therefore I had to book one of the flights arriving the day before.

Is there any "grace" period for MCT, meaning if I call up, would they let me move my LIN-LHR into that early flight on the same day, even if it misses MCT for 5 minutes?