Simplify the following text for a Grade 9 native English-speaking person.
Original: Greater London is divided into 6 zones ; Zone 1 is the most central , with a boundary just beyond the Circle line and Zone 6 is the outermost and includes London Heathrow Airport .
Simplified: Zone 1 is the most central , with a boundary just beyond the Circle Line , and Zone 6 is the outermost and includes London Heathrow Airport .