The same day, time doesn't matter. However, I think you can do it 24 hours before, but I may be mistaken. Traffic on the M4 from Heathrow to London is VERY hectic at 7 in the morning on a weekday. You might be better off taking the Heathrow Express to the Paddington Station then renting a car from there... my 2 cents. I have never taken the tube so I am not familiar - but have always rented cars in London.