How reliable are the National Express Coach bus schedules?
Still looking at a means to get from Liverpool to LHR ?
Hopefully you are not doing this on the day of the flight. ( Unless your flight is after 6PM it is a high risk endeavor)
National Express, whilst reliable, are affected delays as are other road users when there is an incident. By means of example, the M5, a major motorway, was completely closed for 2 hours today whilst the police and bomb squad investigated a suspicious package that turned out to be an old flare. Anybody travelling on this route would have been seriously delayed.
Trains tend to be less delayed.