Unless there is a security issue, no one is going to send someone to hunt through the tens of thousands of bags to look for your bag. This is LHR - you do not believe that everything is organised and automated with manual over-rides do you? They have enough trouble get the teams to load the bags into planes for on-time take off. Furthermore, LHR T3 is hell with various airlines and loads of flights.

You must have known there was a long lay-over in LHR when you booked the ticket. Why are you raising this question now.

If it was a separate booking, then the lesson is not to check your bags through.