I did Stansted - Gatwick and back today. Admittedly it is going away from Heathrow but I got stuck in traffic for about 45 minutes each way due to an accident one side and congestion on the way back. Quite light in the scheme of things.
At 6pm, there is a high demand for the M25 and there from a statistical point of view, more chances of an accident and it takes just one minor incident to cause one Huge f**k up.