The clean driving route is:
Heathrow → M25 clockwise/northbound → M25 J27 → M11 northbound → Cambridge
The question is not really M25 vs M11. From Heathrow, the M11 is the road that takes you up to Cambridge, but you normally need the M25 first to reach it. The M11 runs from east/north London toward Cambridge and ends near Girton/A14, serving Cambridge directly.
Avoid trying to cut through London via A406/North Circular to reach the M11, unless live navigation shows a major M25 blockage. That route is more exposed to urban traffic, lights, bus lanes, and unpredictable delays.
Current check: the M11 is showing clear in the live feed I found, with typical weekday peaks around 07:00–09:00 and 17:00–19:00. National Highways’ current high-priority alerts I found do not show a Heathrow–M25–M11-specific closure, though they do show unrelated major-road incidents elsewhere.
