Delta only flys to London Gatwick (LGW), no flights to LHR. You would have to connect through Atlanta (ATL) or Cincinnatti (CVG). September should be a good time to go.....even mid to late August is OK as most europeans finish their holiday at the beginning of August and schools begin mid to late August. I nonreved to LGW mid August last year and Business class was WIDE open

Enjoy!