On a Monday afternoon in May 2019, I will take the National Express from LHR T3 to Cambridge. I can make a reservation, or not. Making a reservation guarantees a seat. Making a reservation for two hours after the flight's scheduled ETA would allow for delays, but isn't appealing as my flight has very good on-time performance. No reservation allows me to take the next available bus...provided there are empty seats.
Are buses on this route often or usually full?