Any particular reason for the specific routing, you could just fly EK out of LHR and not have the issue if you are indeed connecting. I normally say 2 hours from landing to taxi rank including collecting luggage. The issue you have is needing to check in at DXB for EK and missing checkin times as your possibly in the wrong Terminal ie T1 and EK use T3. If you did mobile boading, again you run it close. You are better to either fly EK or have a stop in DXB and then fly.