If they are both on Delta your LHR flight might go from either the domestic terminal or the new International terminal but if the latter you can stay airside and go on the Plane Train or even walk. You don't have to hang around getting the shuttle bus like people who begin at Atlanta for an international flight.
All the same, sounds optimistic to me but I have never tried to make that journey. When does the gate close?
The other thing is, I only see Delta flying to LHR at 10:55 - which is good as it would give you another 10 minutes - but maybe there's a seasonal change in the schedule.
Sorry not to be definitive. Did you book directly with Delta?