I work near LGW and when I head north on a Friday after work, it takes about 2 hrs to get to LHR. I leave work at 4pm and get to the M40 around 6pm. It's just 10mph crawling.

Maybe try the train to London Victoria, tube to Paddington then then HEX to LHR? Reckon you could do it in 90 mins assuming no major probs with the circle line.