Your only option is to spend the night in London on your way back I'm afraid - nobody will get you to LHR early enough to catch your connection by train or air. I agree you should definitely consider BMI though - at the moment you can get a web-only fare of £9 + tax each way. Of course, the drawback with this e-ticket fare is that you won't have a chance of persuading them to check your bags all the way from ATL to CDG.