In FS2004, when you select flight planner, you choose your airports, then you choose the type of flight path you will be taking. You get an option of GPS, High Altitude airways, low altitude airways etc. What do you classify as high or low altitude.
How high should I fly if I was going from Luton to Manchester on a 737 and on 747?
I don't know. How far is that? Is it like 10 miles, or 1000 miles? 😉
In general, jets will be operating at high altitudes, and prop planes won't. In almost any plane, the higher you fly, the less fuel you consume and the faster you can go (in ground speed, not indicated airspeed).