As others have said, there isn't an alternative that is that much better. My suggestion would be to go to Gatwick the night before and stay overnight in a hotel. That way you can time your trip so that you avoid pass through, or around, London in the early to mid evening, after the rush hour when there's not too much traffic. If you left Nottingham around 6pm, you would be going around the M25 at 7:30 or so and in your Gatwick Hotel by 9pm.
We always go to Gatwick the night before, so we don't have to worry about traffic, breakdowns or punctures. We have a leisurly evening meal at the hotel (early check-in if you are flying Virgin) go to bed then a relaxed walk over to check-in (we usually stay at Hilton Hotel inside the terminal - but there are other hotel choices) We have breakfast in either the hotel or in the terminal depending on the time of our flight.
It's adds an extra day to our holiday!!