Bath to Heathrow with National Express. Are there delays?

Take the earlier bus, better an hour or two having coffee/breakfast at the airport than missing your flight. What does your airline recommend, it is usually 3 hours for an transcontinental flight?

They recommend to be one hour and a half or two earlier. It is in the National Express website where it is reommended to plan the arrival at least 3 hours earlier.
I do not want to put my flight at risk, but with the earlier bus I would arrive 4 hours and a half before the flight...

Thank you for your advises.
I am going from Bath to Heathrow by bus, and then, I take a flight to Bilbao (Spain) with vueling. I have asked in the bus station, and they have told me also that is better to go in the earlier bus, although I may have 4 hours of waiting in the airport.

I'm confused - is this your going home flight or a domestic one around Europe somewhere? Which airline?

We go to UK often and at times travel by bus to the Midlands or some times to the west counties. The bus is cheaper than Train which is cheaper than air. National express is usually close being on time. There is one caveat though.
Motorway traffic / road work delays and as the British say "motorway incedents" (accidents) on the motorways that can cause long delays or slow downs. we usually give ourselves at least a two hour safety net in addition to the normal travel time. Knowing full well that often this results in being at the airport long before we need to but at least we are there rathe than sitting in the bus fretting. L/M