The cheapest flight can become the expensive option when it departs before the first convenient train or lands after the last one. Always plan the airport journey together with the flight time.
Check the first and last useful service
Do not check only whether a train or coach runs that day. Check whether it runs early or late enough for your terminal, check-in and security time. The first train can be too late for an early departure even when the airport has a good daytime rail link.
Coaches fill many gaps
Airport coaches run through the night on selected routes and are especially useful for Stansted, Luton, Gatwick and Heathrow. Frequency varies, so verify the actual timetable for the travel date rather than relying on a general '24-hour' label.
Sometimes a hotel is rational
For a very early flight, an airport hotel can cost less than a long taxi ride and reduce the risk of a missed departure. This matters most when the cheaper airport is far from where you are staying.
A return ticket may not reserve the return coach
Do not assume that having a return coach ticket guarantees a seat on the departure you want. A flexible return can still leave you behind if the coach is full with passengers holding reservations for that time. This matters most for very early airport runs, when missing one coach can mean a long wait or a missed flight. For Luton, see the timed-coach practical tip.
Check live times before you leave
For an early or late journey, check live information rather than relying on remembered service hours. Use National Express for coaches, National Rail for mainline trains, TfL Go for London transport, and Luton DART for the station-to-terminal link.