Apologies if this is obvious but just to add gates/doors are closed at 20 mins before departure time. So meeting conformance does not necessarily mean mean not missing the flight. For example a bag sent for a random secondary scan and then a departure from T5C gates would quickly eat into that 15 mins.
And for completeness bag drops close 60 minutes before long haul and 45 minutes before short haul flights. A number of years ago I did get a bit of flex on the latter deadline at Gatwick, but I wouldn’t recommend it!